Detecting inorganic arsenic below WHO threshold limit; A comparative study of various sensors

ABSTRACT The issue of heavy metals contamination of environmental segments is becoming serious day by day with industrial and electronic developments. Economic growth and environmental issue are parallel in many aspects. For instance, increased number of vehicles and rapid industrialisation are indicators of economic growth. Economic run is important and shall continue because it is associated with global needs while environmental pollution is otherwise. Necessary steps are required to be taken to control the release of unwanted chemicals including heavy metals and arsenic to the environment. Arsenic is present in water both in its organic and inorganic form. Among these forms inorganic arsenic (As(III) and As(V)) are very toxic. WHO has recommended a threshold limit for As(III) in drinking water i.e. 10 ppb (equivalent to 10 μg L−1 or 133 nM). Sensing of arsenic has been made possible due to several techniques with a wide range of detection limit. Herein very recent As(III) sensors with detection limit below 10 ppb, mechanism of detection at certain instances, interferants during measurements and future goals have been discussed. Further, the review article covers literature mostly from 2015 to 2021.


Introduction
Environmental pollution as a result of industrialisation is a matter of concern which has adversely affected the environment and its biota [1].There are certain chemicals like heavy metals which are persistent [2], bioaccumulate in food chain [3,4] and cause severe health issues [5].The term 'heavy metal' has been remained controversial and currently we have proposed a definition based on three criterions, where arsenic does not meet all the conditions and does not come under the term [6], no doubt arsenic (focus of this article) is non-essential and extremely toxic to life.Arsenic contamination of ground water is one among the largest environmental health crises.More than 300 million people in one-third of the world are exposed to arsenic poisoning [7].Arsenic is a metalloid in nature, classified as Class-A human carcinogen and inters into human body via As-contained water and edible crops [8,9].It exists in water in two common oxidation states, arsenite (inorganic As(III)) and arsenate (As(V)), which are toxic to human health.As(III) is 60 times more toxic in comparison with As(V) and its organoelement derivatives [10].To address the issue of toxic elements and other pollutants, at the first instance they need to be detected in environmental bodies followed by effective removal or detoxification.So far various analytical methods are available to detect and quantify toxic elements in environmental samples.Speciation of inorganic arsenic has been effectively carried out by Nazir and co-workers in real samples [11][12][13][14][15].The sensing techniques with regard to heavy metal ions range from single molecule detectors [16] to modern electrochemical sensors.Several review articles have been published in recent years wherein the sensing devises and techniques have been the focus of consideration.These sensors are based on; calixarene [17], whole-cell biosensors using eukaryotic microorganism [18], polymer-based and other electrochemical [19][20][21], optical fibre [22], polymer nanocomposites [23], atmospheric pressure discharged plasma [24], nanofibers in colorimetry [25], nanomaterial based [26], screen-printed electrodes [27], graphene based [28], glutathione probe [29], carbon nanodots [30], carbon nanotubes (CNs) [31], nanozyme-based [32], quantum dots [33], DNA based electrochemical sensors [34][35][36] and Metal and metal Oxide nanoparticles based [37].These developments and efforts are continuing towards translation from conventional laboratory analysis to field-portable analysis for detection, quantification and effective removal of heavy elements [38][39][40][41].Arsenic detection on arsenic material, a study in near past was based on Ag 3 AsO 4 as sensitive membrane for detection of As(III) in water is a move towards circular economy and needs serious considerations to enhance sensing capacity of the material for field analysis [42].A sensor possesses various characteristics to be used for sensitive and selective determination of a species under certain conditions, where limit of detection is one of the criteria to be below WHO guidelines.These sensors can be laboratory based or field portable and suitable for on-site analysis, they can be rigid or flexible.
A comprehensive literature review reveals that efficiency of sensors towards an individual metal ion has not been addressed with in-depth comparison.The aim of this review article is to focus on recent sensor platforms with detection limit, less than 10 ppb.They are polymer based, metal and metal oxide nano-particles, biosensors and those developed from ionic liquids, Fig S1 .Salient features of these sensors have been discussed, basic chemistry and some mechanistic approaches are the focus of this review.

Sensors for arsenic detection
Arsenic is a nonessential element present in ionic form in water and is extremely toxic.Its intake through any route is toxic and leads to the development of cancer, diabetes and cardiovascular complications [43].Its detection and subsequent removal from water is very important.A large portion of world population is exposed to arsenic contaminated water, 35-77 million in Bangladesh only [44].The maximum concentration value of arsenic, set by WHO is 10 ppb, in some places of the world a concentration as high as 5000 ppb has also been reported.Arsenic is added to water by natural and anthropogenic sources.A number of techniques for detection of this deadly toxic ion are in field.Surface-enhanced Raman scattering (SERS) is very effective technique for chemical and biosensing of arsenic species at ppb level [45].A very recent review deals with various sensors with special emphasis on the surface morphology of the material and their mechanism of detection.Authors have described nano-enabled sensors for field applications, associated problems and possible solution to some issues such as interference with coexisting ions [46].Calorimetric detection of the same species in water samples has recently described, where nanomaterials are used for the purpose of detection at ppb level in drinking water [47], several articles also deal with simultaneous detection of ions including As(III) [48,49].
Exposure of people to unsafe level of As(III/V) in drinking water in most parts of the world is a serios problem.The development of low cost, on-site sensor for detection of As (III/V) is need of the day.During sensing of arsenic the main interferants explored so far are; Cu(II), Hg(II), Pb(II) and hydrogen generation on the electrode surface.During designing a sensor, it must be kept in mind that a sensor must preferably be interference free.The sensing depend upon various properties and sometimes the sensitivity can be improved with increasing porosity while keeping the material unchanged [50].Several characteristics have to be in mind during designing the sensor like, sensitivity, repeatability, reproducibility, selectivity, easy miniaturisation, easy fabrication, commercially viable, sustainable, field compatible and stable under various operating conditions.

Polymer nanocomposites (PNCs) based sensors
The role of nanocomposite material in sensors development technology for sensing toxic species in environmental samples are described in full details and mechanism of removal or detoxification as a surface phenomenon has been very nicely addressed, interested readers are referred to the article [51].Removal of As(III) and As(V) has been carried out by polyaniline/iron(0) (PANI/Fe(0)) system [52].The removal capacity of the material for As(III/V) was 232.5 and 227.3 mg/g, respectively.The efficiency was shown under conditions, pH 7 and ambient temperature, it was further explored that coexistence of some anion (HCO 3 -, SiO 3 2-, SO 4 2-) had no adverse effect on the removal efficiency but it was substantially reduced by the presence of NO 3 − and PO 4 3-.The modified PANI has also been extensively used for sensing of the target ion.A PANI thin film electropolymerized on GCE followed by deposition of Au NPs to give Au NPs/PANImodified GCE for detection of As(III).The material was fabricated in the absence and presence of additive KI (potassium iodide as a source of iodide ion).The electrode material prepared in the presence of iodide showed higher activity (selectivity towards As(III), DL 0.4 ppb) [53].The composite, Au-PANI-Fe-CNFs was obtained by the formation of PANI nanosheets array on the electronspun Fe-CNFs substrate followed by selfdeposition of Au NPs.Size of Au NPs in the range 14-26 nm were uniformly deposited on the polymer surface.Incorporation of Fe in CNFs improves the As(III) deposition on the electrode surface.The electrode displayed a wide linear range 5-400 ppb with DL 0.5 ppb, in real water samples towards As(III) detection [54].The interference of Cu(II) during experiments was masked by addition of EDTA.PANI NF/carbon dot nanohybrid is an efficient fluorometric sensor (DL 0.001 ppb) [55], see Table S1 for comparison.The sensing ability of material can be affected by surface area, which is probably larger in this type of electrode material to give excellent results during real samples analyses.Material based on analogous semiconductor polymer, Au@PPy (PPy polypyrole) was developed as alternative to eliminate Cu(II) interference during As(III) detection.Under optimised condition of 120 s deposition time and −0.4 V potential, the limit of quantification was 0.2 ppb.The sensing material is superior to some of the aforementioned sensors and can be used for As(III) detection with the desired sensitivity [56].Au NPs functionalised PPy nanowires (Au NPs/PPy NW) for detection of As(III) in water are very effective material.The PPy NW was fixed on electrode surface followed by functionalization with Au NPs.The sensing performance of the modified electrode towards As(III) was very excellent.The sensitivity of the sensor was 63.13 mA/mM (0.8417 mA ppb −1 ) and DL was ultralow, 0.37 ppb (0.005 mM) in real water samples [57].Nanocomposite of Fe 2 V 4 O 13 and PPy on SPCE is acid resistant and was used as one-shot disposable sensor.It exhibited high sensitivity with a wide linear range 0-500 ppm and DL 0.3 ppb [58].The nature of the sensors of this type being flexible are supposed to be used for future quantification of As(III) ion.Some sensors are very simple in fabrication and efficient in sensing such as gold nanorods (GNR) chemically conjugated with poly(ethylene glycol) methyl ether thiol (PEG-SH) followed by dimercaptosuccinic acid (DMSA).The colorimetric material was able to change colour with the addition of As(III) and As(V) ions.The colour change in paper based sensor is because of GNR aggregation as a result of arsenic complex with DMSA [59].Some other polymers modified with some metal or metal oxide NPs like α-MnO 2 doped polydopamine (α-MnO 2 @PDA) are also in use for the development of publicly and scientifically acceptable sensors.Surface of the electrode was modified by coating with the polymeric material and was used for detection of As(III) at very low DL 0.13 ppb.During the fabrication process Nafion was employed due to its excellent film forming properties to firmly paste the metal oxide modified layer on to the electrode surface.This sensor possesses properties of reproducibility, sensitivity, specificity and robustness for practical applications [60].

Graphene oxide (GO) based sensors
Amine functionalised graphene oxide (H 2 N-GO) decorated Au microelectrodes possesses certain features to detect As(III) at ppb level (DL 0.162 ppb).The synergistic effect of H 2 N-GO and Au microwire was utilised for detection of As(III) ion.The electrode was found robust, reproducible, sensitive (130.631μA ppb −1 cm −2 ) and selective towards As(III) in water samples under optimised conditions.No obvious interference from Cu(II) or any other species was detected during experiments.The material is quite promising in designing interference free electrode for environmental monitoring [61].Chemical modification of graphene with Pt nanoparticles was carried out for the purpose of As(III) sensing.Reduction of GO and H 2 [PtCl 6 ] in the presence of ethylene glycol was able to show linearity in concentration range 10-100 nM with detection limit 1.1 nM (0.083 ppb).The proposed sensor was able to detect As(III) in real water samples with no prominent interference from other cations and anions present in the same sample of lake, agricultural soil and borewell water [62].Au NPs/rGO/GCE was used for inorganic arsenic, As(III/V) in environmental samples in neutral solution at room temperature.The interfering effect on experimental results was studied with several metal ions such as Cd(II), Pb(II), Cu(II), Mg(II), Ca(II), Ni(II), Fe(II), Fe(III) and Ag(I).Among these ions Ag(I) was found to alter the sensitivity of the sensor due to greater affinity of this ion with Au on electrode surface.It has been suggested that during designing such an electrode, more Au may be deposited on the surface to avoid possible interference due to Ag contamination [63].Au NPs/rGO/GCE was perpetrated in a green synthetic way without using any reducing or stabilising agent.The sensor was tested under normal experimental conditions and was found efficient to detect As(III) below WHO permissible concentration.The interference of Cu(II) and other inorganic cations, Al(III), M(II) ions (M = Pb, Mn, Hg, Zn and Cd) was found negligible, which makes the sensor suitable for fast analysis of As(III) ion in real water samples [64].Au NPs deposited on CNTs via in situ reduction of HAuCl 4 by NaBH 4 and immobilised on GCE was tested for sensing of the target ion.The ASV was performed for determination of sensing applications where the detection limit of the sensor was 0.1 μg L −1 (0.1 ppb) and sensitivity ≈ 2000 μA μM −1 (SWV) with deposition time of 2 min [65].The MPS-Au (3-mercapto-1-propane sulphonate) was fabricated with multilayers of PDDA (poly(diallyldimethylammonium chloride)) and Au NPs by LbL (layer-by-layer) technique.The architecture with five bilayers was efficient in comparison with bare gold electrode in sensing towards As(III).The electrode was poison tolerant and the sensor was able to detect As(III) below WHO limit [66].The GHS/DTT/Cyc/Au NPs gives negligible response to Fe(III), and M(II) ions (M = Pb, Fe, Zn and Cd) but good response was given to Hg(II) ion.Incorporation of another chelating ligand, 2,6-pyridinedicarboxylic acid (PDCA) which make stable complex with Hg(II) thus suppressing its interference during As(III) detection.Compared to common colorimetric method the DSL (dynamic light scattering) assay is employed to know variations in the particle size, it was applied to As(III) detection and the sensitivity as low as 10 ppt was observed [67].A polycrystalline gold nanocomposite (sub-BT/Au) used for the purpose of As(III) detection was very impressive with regard to negligible interference from Cu(II) ion.The fabricated electrode is robust, sensitive towards As(III) ion even in the presence of Cu(II), linear response was recorded up to 15 μM, sensitivity 27.01 ± 0.01 μAcm −2 μM −1 and DL 0.28 ppb.The sensor is promising for real environmental samples without interference from copper or any other common cations [68].Electrochemical assay for As(III) ion using SWASV was performed on TTCA/rGO modified gold electrode (trithiocyanuric acid deposited on reduced graphene oxide).Two material TTCA and rGo are held together with the help of noncovalent interactions, π-π and the surface is made suitable for As(III) adsorption.The detection (DL 0.054 ppb), sensitivity in real water samples and smart combination of thiol-rich TTCA and rGO material are some of the attractive properties of the sensor [69].The Fe 3 O 4 -rGO (reduced graphene oxide) based sensor was used in some studies for sensing As(III) ion in water samples.These sensors are capable to detect the target ion in trace amount, the sensitivity being recorded in the range of 0.142-2.15μA ppb −1 and limit of detection was 0.10-1.19ppb.The sensors are selective in sensing towards As(III) ion.The method of fabrication of the sensor influences the resultant efficiency.The DPASV was found efficient in terms of sensitivity, detection limit and stability.After 30 successive cycles, the sensor exhibited up to 93% stability for aqueous samples.The sensor was tested for lake water at room temperature, the results were compared and found in close agreement with ICP-MS (inductively coupled plasma mass spectrometry) results [70].The same material was synthesised in a separate study and was tested for the same metal ion.The interference of other metals in oxidation state +2 was negligible, which indicates high selectivity of the material towards As(III) ion.The characteristics like sensitivity (0.281 μA ppb −1 ), reproducibility and stability of the sensor over concentration range 0.01-5050 ppb (DL 0.12 ppb) make it appropriate for practical environmental monitoring [71].In a separate study the authors found Fe 3 O 4 /rGO-based sensor sensitive, stable and additionally they explored it against interfering ions [72].
During experiments, the target ion (10 ppb) in presence of 100 ppb M(II), SO 4 2-ions and 200 ppb anions (acetate, chloride and bromide).The Au NPs/rGO/GCE material is capable to detect As(III) at detection level 0.2 ppb in real water samples.The efficiency of the sensor was adversely affected by the presence of Cu(II) ion in the same solution [73].Cost effective noble metal free material, rGO/MnO 2 nanohybrid on GCE was fabricated for As(III) detection in water samples.This type of material beside economic viability, possesses favourable sensitivity (0.176 μA ppb −1 ) and selectivity.Possible interference from co-existing cations was eliminated by adding EDTA or cation exchanger before quantification of the target ion [74].Schiff bases contain some potential coordinating sites in addition to the imine -C = Ngroup, if used for modification of material, it certainly enhances the sensing efficiency of the material.In a recent attempt Schiff base obtained from diethylenetriamine and 2-hydroxy-4-methoxybenzophenone, was used for functionalization of GO.The Schiff base (SB) functionalised GO was used for preparation of nanocomposite, SB@SiO 2 @GO@ITO (ITO = indium tin oxide) as working electrode towards As(III) sensing in water samples.This type of material is cheap, non-toxic, non-metallic and very sensitive (DL 156 pM ≈ 0.012 ppb) towards the target.The Schiff base functionalization was found effective in As(III) arrest through possible complexation [75].Extensive research in this field has materialised various sensing material which are commercially available (trade name DRP-110GPH.DropSens, as an example) for arsenic determination below WHO limit.This material needs technical personnel for optimisation of working condition during testing environmental and biological samples [76].Further research is required for development of sensors which must be interference free, selective, stable and shall work preferably under any conditions in order to eliminate complications during sensing and to save time as much as possible.GO modified with coordination complex of ruthenium, [Ru(bpy)] 2+ and deposited on SPCE ([Ru(bpy) 3 ] 2+ -GO/SPCE) shows excellent detection capacity.The sensor was able to detect As(III) and As(V) with no potential interference due to coexisting ions except Hg(II) in 20-fold excess.Linear range (0.08-16 μM), selectivity, repeatability and reproducibility of the sensor was good with a standard deviation 2.84 for As(V) and 2.67% for As(III) [77].GO and Graphene containing sensors discussed here, give detection limit in the range 0.01-4.4ppb, lower DL was recorded for GHS/DTT/Cyc/Au NPs (0.01 ppb) [67] and SB@SiO 2 @GO@ITO (0.012 ppb) [75] and highest for Au MPS/(PDADMA-AuNPs) 5 (4.4 ppb) [66].It is clear that organic compounds like Schiff bases have potentials to be used for modification of the electrode material for better results.

Metal oxides (MOs) based sensors
Metal oxide nanoparticles have attracted larger interest for their use in the field of sensors.They possess certain intriguing properties such as large surface area, catalytic ability, and very good photophysical and electrochemical performance.MOs are integral part of sensor technology for determination of persistent toxic elements and other materials [78].This topic deals with those metal oxides which do not fit under any of the above topics and where the sensing of the target ion is attributed to the presence of metal oxide within the material.Several of the metal oxides have been used during modification of GO, polymer or carbon material and most of them are discussed in their relevant sections.There are plenty of reports where metal oxides have shown sensing efficiency below the admissible limit of WHO.They are briefly outlined hereunder; A recent study on Mn 3 O 4 nanoparticles opens new windows based on very good results of the material.The octahedral shaped Mn 3 O 4 NPs are ultrasensitive and selective in As(III) detection and quantification.The adsorption of As(III) on the surface causes reduction and subsequent release of Mn 2+ from the surface to produce active sites which enhances the oxidase-mimicking catalytic activity.Final material after adsorption show absorbance as 450 nm, the DL is 1.32 ppb which falls below the WHO guidelines and make the material useful for water analysis [79].
Agglomeration of magnetic nanoparticles, Fe 3 O 4 is a probable hindrance towards poor performance of the material in the field of sensing.This issue can be intelligently resolved by the use of ultrasound and have attracted attention in current and future researches.In comparison with stirring dispersion, the particle dispersed with the assistance of ultrasound was very fruitful.The efficiency of Fe 3 O 4 @Au NPs in sensing was 7-fold higher for total arsenic As(III/V), linear calibration range was 0.01-3.0ppb and DL was 0.2 ppb [80].This type of material was employed for As(III/V) enrichment and the interference of ions like Cu(II), Ni(II) and Co(II) was determined to be effective during hydride generation and not in enrichment processes [80].Fluorine-doped cadmium oxide (F-CdO) nanoculiflower like material were fabricated in a thin filmbased working electrodes.The material through its electroanalytical property (without the use of arsenite oxidase or arsenic reductase enzyme) was able to detect As(III) with DL 4.55 � 10 À 3 ppb and sensitivity 5.747 � 10 À 3 μA ppb −1 [81].The surface morphology of Fe 3 O 4 reveals that this type of metal oxides are good adsorbents.Its composite with Au (Au-Fe 3 O 4 ) is dumbbell-shaped and its SPCE modified electrode is an efficient As(III) detector with DL 0.0215 ppb and excellent selectivity 9.43 μA ppb −1 .The redox cycle, Fe(III)/Fe(II) on Fe 3 O 4 surface and redox reaction of As(III) has been confirmed during the study.This type of material encourage research towards the use of cheap redox active material as As(III) or toxic metals sensing platforms [82].The issue of interference is quite serious and it greatly hinders the detection of As(III) in many instances.Among interferants, Cu(II) is the most notorious and several strategies have to be adopted for precise detection of As(III) in water samples.There are some materials which exhibit anti-interference activity like red-mud-reduced graphene oxide (RM-rGO).The NC material is exponentially selective, sensitive (2.49 μA ppb −1 ) towards As(III) and has a very low DL 0.07 ppb.The efficiency of NC is due to the adsorption proficiency of Fe 2 O 3 , present in the red-mud and additionally the process of detection is supported by enhanced electron-transfer kinetics due to rGO [83].The principles of catalytic redox couple Fe(II)/(III) induced by Fe(0) in Fe(0)@Fe 2 MnO 4 have been considered in a recent studies with outstanding activities [84].The sensor based on nanocomposite, Au NP/Co 3 O 4 (Au NP on porous cobaltous oxide) on SPCE behaves as anti-interference material towards As(III) sensing.The lower sensitivity 12.1 μA ppb −1 and lower DL 0.09 ppb was observed during experiments for sensing As(III) in water and human serum samples [85].Nanocomposite SrTiO 3 /β-CD modified GCE is extra stable, has good sensitivity and reproducibility under optimised conditions.The target ion As(III) was determined (amperometrically) with DL 0.02 μM [86].
The limit of detection for sensors fabricated from various material ranges from 1.5-4.55� 10 À 3 ppb.The highest DL was reported for SrTiO 3 /β-CD/GCE [86] and lower value was observed for F-CdO electrode [81].Here the functionalised CdO gives better results but the toxicity inherited by Cd itself can be a question while using it as sensor.Extra care shall be taken while disposing the electrode material.On the other hand, the low cost of the material has greater potentials to be used in the sensor technology.

Metal nanoparticles (MNPs)
Nobel metals are ideal for fabrication of sensing devises because of their unique optical behaviour [87].The material containing these metal ions are highly dependent on the environment around the metal ion and slight change therein can cause a wide difference in their properties.Ag and Au NPs are widely used due to their least toxicity, inert nature, easy fabrication and fast response, which enable them efficient candidates for environmental and public health monitoring [88].Metal nanoparticles are widely used in sensing technology for quantification of various species, including As(III) in food samples [89,90].
Here only those material will be discussed which gives sensing of the target ion in concentration below or in the range of WHO guidelines.
Metallic Ru NPs on DCE were formed at electrodeposition potential of −0.75 V.The modified electrode was used for As(III) sensing where an increase in charge transfer resistance with increase in As(III) concentration was observed spectroscopically.The interference of Cu(II) was critically examined, there were no indications of interference from this cation.This characteristic of the sensor eliminates certain hectic practices such as preconcentration step in stripping voltammetry, longer time and stirring.Detection and quantification of As(III) by using DPV takes few minutes under mild conditions for real environmental samples.A DL 0.1 ppb, reproducibility of 5.4% and sensitivity of 2.38 nA ppb −1 was reported for this sensor which are promising for practical applications [91].Au NPs/GCE in HCl solution with the help of highly sensitive ASV was applied for detection of As(III).The sensor was efficient at its sensitivity 0.32 mA cm −2 μM −1 and detection limit 1.8 ppb, while studying the effect of anions, the sensitivity was higher in the presence of I -ion [92].Au NP electrochemically deposited on GCE show ultralow detection limit towards As(III) sensing with achieved sensitivity 95 μA μM −1 .The DL was excellent by using LSV (0.0096 ppb) while SWV was not that much efficient (0.014 ppb).During studies Cu(II) was found to interfere normal sensing capacity of the sensor [93].The problem of interference of Cu(II) ion during As(III) sensing was followed by the same authors.They designed two types of Au NPs modified electrodes.Au GCE and basal plane pyrolytic graphite (BPPG) were separately used as substrates for deposition of Au NPs.The behaviour of Cu(II) and As(III) was studied on these electrodes and it was found that the interference during As(III) sensing can be reduced by using Au NPs electrode [94].In presence of 2 μM concentration of Cu(II) in the solution, three electrodes Au-macro, Au NPs/GCE and Au NPs/BPPG exhibited 0.024 ± 0.005, 0.006 ± 0.001 and 0.234 ± 0.05 μM, respectively.In real water samples As(III) was determined with the help of anodic stripping voltammetry (ASV).Au NPs deposited on carbon fibre (CF) ultramicroelectrode was able to allow detection in the range 5-60 μg L −1 under optimised conditions.The determination process of As(III) is rapid, with better repeatability, selectivity, lower DL (0.9 μg L −1 ) and reasonable sensitivity (0.0176 nA μg L −1 ).The results were compared with those obtained by hydride generation atomic absorption for the same samples and were found in close agreement with each other [95].The material of this sensor is less expensive, the sensor is reliable and may be used for determination of inorganic arsenic in real environmental samples.Au NPs deposited on boron-dopped diamond (Au NPs/BDD) was designed for determination of total arsenic in water samples (As(III/V)).The As(V) was reduced to As(III) in the presence of thiosulphate in 1 mol L −1 HCl solution.The SWASV was used for determination of arsenic in the linear range 0.1-1.5 μg mL −1 .The limit of detection was 20 ng mL −1 , the results obtained in repeated experiments were precise at low working potential and sensor-tosensor reproducibility, the results obtained were validated by using ICP-OES (inductively coupled plasma-optical emission spectroscopy) [96].During studying interference of various species in samples, Cu(II) adversely affected the results which was eliminated by addition of ferricyanide.Reusability after 5 cycles, careful and technical electrodeposition of the Au NPs on BDD electrode are issues to be resolved in future researches.Au NPs (30-60 nm in diameter) was deposited on the surface of glassy carbon microsphere and a powder product was obtained, the modified powder was characterised and used for electrochemical determination of As(III).The sensing material was capable to determine ppb level concertation of As(III) in water samples [97].The interference caused by Cu(II) was masked by ammonia and Au NPs modified GCE was used to quantify As(III).The interference was well suppressed under optimised condition of pH 3, deposition potential −600 mV and deposition time 60 s.The As(III) in DL 2.4 ppb was detected in industrial waste water and the results were validated by ICP-MS (2.3 ppb) [98].
Glucose functionalised Au NPs upon addition of As(III) changes its colour from red to bluish and can detect As(III) selectively in linear range 1-14 ppb and DL 0.53 ppb [99].An exfoliated graphite electrode (EG) modified with composite of Co NPs and rGO was used for quantification of As(III).Electro-active surface area for electrocatalytic activities of Co NPs/rGO EG was well improved and detection of As(III) in standard samples with DL 0.31 ppb was made possible [100].Functionalised silicon (3-aminopropyltriethoxysilane) nanoparticles (f-SiNPs) deposited on SPCE reveals anodic peak current proportional to the concentration of As(III) in water samples.A wide range of 5-30 ppb with DL 6.4 ppb was obtained for real water samples.Among interferants (Ni, Zn, Pb, Hg, Cu, SO 4 ) Cu(II) strongly affected the sensing capacity [101].The material was improved by incorporating Au NPs for better and interference free detection of the target.Electrochemical detection of As(III) used linear sweep anodic stripping voltammetry (LSASV) was made possible at its lower concentration, at ppb level.The silica/gold nanoparticles (Si NPs/Au NPs/ SPCE) electrode does not show uniform distribution of particles on the surface of the electrode [102].The sensing electrode was used in 1 M HCl, and deposition time was optimised to 30 s at deposition potential of −0.4 V. Linear correlation of 10-100 ppb with DL 5.6 (lower than the Si NPs/SPCE) with no potential interference in the presence of coexisting ions like Pb, Ni, Zn, Hg and Cu.Furthermore, the sensitivity and reproducibility of the sensor is suitable for detection of arsenite in environmental samples.Sparked Au NPs instantly prepared from eutectic Au/Si alloy is suitable for detection of As(III) in water.The sparked nanoparticles were prepared on the basis of evaporation-condensation process by using Au and Au/Si (97/3 wt%) alloy (represented as eAu NPs/SPE).In comparison to Au NPs/ SPE, the eAu NPs/SPE is 5 times more sensitive, fast in detection with a wide range (0.5-12 ppb) and with a low detection limit of 0.022 ppb.The sensitivity was affected by the presence of Cu(II) where the authors have recommended the application of ion exchange resin before sensing As(III) ion [103].Natural bentonite grafted with silane is a nanocomposite and can work as electrode with enhanced surface area.Redox phenomenon occurs in a single step in acidic medium (pH 2).The As(III) was determined in real water samples with linear detection range 0.5-20 ppb, DL of the sensor was calculated to be 0.0036 ppb with relative standard deviation <4%.In the presence of coexisting cations and anions the sensing efficiency was affected by Cu(II) and Mn(II).The sensor material is very cheap with very reasonable DL and is suitable for Cu/Mn free water or elimination of these interferants shall be carried out before quantification of the target As(III) ions [104].Gold nanotextured electrode (Au/GNE) was developed on a simple gold foil via electrochemical redox sweep in a metal free solution.Linear range for As(III) determination of the material was reported to 0.1-9 ppb, sensitivity 39.54 μA ppb −1 cm −2 with a low detection limit of 0.1 ppb.The electrode was able to determine As(III) with no interference from coexisting cations in the solution [105].A hybrid material, Au NPs/CeO 2 -ZrO 2 nanocomposite modified GCE works under natural pH conditions (pH 8) in real water samples.The electrode exhibited low DL 0.137 ppb and high sensitivity 20.674 μA ppb −1 with good anti-interference performance [106].
Ag NPs are accessible through green synthetic approach in the presence of stabilising agent and a reducing agent (β-cyclodextrin and ascorbic acid).The material were easily fabricated as disposable As(III) detector in ground and river water samples.The sensor was active with a wide linear range 13.33-375.19nM and sensitivity 180.5 μAμM −1 [107].Sensitivity of Au NP/GCE was 1.04 AM −1 in comparison with higher value obtained for Au NPs/BPPG 3.52 AM −1 .Plasmonic silver chip (pAg chip) is portable, sensitive and effective for As(III) detection.The chips were fabricated through seed mediated method to grow the Ag nano-Island films (Ag-NIF) on a suitable substrate.A fluorescent active dye Cy7.5 was immobilised to enhance fluorescence signal up to 10-folds.The pAg chips were able to detect As(III) below the limit as recommended by WHO [108].
Metal nanoparticles exhibit a very broad detection limit towards As(III) ion, 6.2-3.6 � 10 À 3 ppb.The poor detection limit was observed for Si NPs/SPCE [101] while eAu NPs/SPE [104] gives excellent value towards traces of As(III) in water samples.The potentiality of gold nanoparticles in this class of sensors is very evident.The particle size and method of preparation under optimised conditions shall be strictly observed.

Carbon quantum dots (CQDs)
CQDs belong to carbon nanostructure family, they are physically and chemically very stable, less toxic and possess excellent photoluminescence properties.Functionalization of CQDs is very easy and fast, some are water soluble and due to non-toxic nature, these materials are better alternatives for detection of certain environmentally notorious cations.The materials have been extensively studied for the purpose of sensing various species through different analytical methods [109][110][111][112][113][114].Some of the materials which show As(III) sensing within the limit of WHO guidelines are enlisted here.
ZnO QDs are fluorescent in nature and based on the sensitivity of this technique any change in fluorescent intensity (fluorescent quenching or enhancement) can lead to useful applications and information.Fluorescent quenching of ZnO QDs upon addition of arsenic compounds was utilised as sensing platform.Quantification of As(III) and As(V) was successfully achieved at ppb level.This type of sensors bear potentials to be used for environmental monitoring during onsite measurements (DL 27 ppb for As(III), 7 ppb for As(V) and 28 ppb for As (total)) [115].Since the limit of detection of this material is higher therefore, improvement in its sensing capacity was direly needed.Thiol functionalisedcarbon quantum dots (HS-CQDs) were prepared by microwave pyrolysis of citric acid and cysteamine followed by functionalization of ditheritheritol.The surface of the S-rich material was used for selective detection of As(III) in water samples.CQDs of uniform size (ca 5 nm) were confirmed to have SH functions and they were found to enhance fluorescence intensity upon addition of the arsenite in 5-100 ppb concentration range.The detector was found to possess DL 0.086 ppb with good reproducibility and high selectivity and sensitivity.The interference of common coexisting anions, cations and other inorganic arsenic was studied and found negligible [116].The CQDs-MnO 2 nanocomposites can be characterised by the fact that blue fluorescence of CQDs is quenched by MnO 2 nanosheets due to FRET (fluorescence resonance energy transfer).As(III) causes decomposition of MnO 2 , which results into FRET termination thus leading to detection of As(III) ion.A low detection limit of 1.26 ppb (16.8 nM) and a linear concentration range up to 200 nM was recorded [117].Fluorescent colorimetry test papers are promising in the field of medicine, food and environmental science for naked eye detection of various species on the basis of variations in colours.A dosage sensitive fluorescent colorimetry test paper was found responsive to trace amounts of As(III) in water samples.The QDs (red) were modified with GHS and DTT (for structure see supporting file) to obtain super sensitivity towards As(III) ion.The material through a serial of colour change was able to detect As(III) below 10 ppb.The detection limit was found 1.7 ppb which makes the material quite suitable to be used for real time applications [118].Sulphur-doped CQDs obtained through microwave pyrolysis of citric acid and sodium thiosulphate can be sued for detection of As(III) and glutathione (GHS) with high sensitivity.The sensing ability of the material was governed through colour change in the presence of As(III) and GHS.Detection limit for As(III) was as low as 32 pM with sigh specificity in the presence of other metal ions [119].The GHS passivated on the surface of CDs, gives excellent optical properties and water solubility.The material is sensitive with 'turn off' fluorescence response with dual detection of As(III) and ClO − .Fluorescent quenching was observed and detection of As(III) within the desired level was sensed, 2.3 nM [120].
Lead sulphide quantum dots and L-methionine (Meth) as capping agents are active in the same way as some of carbon quantum dots.The fluorescent probe is sensitive towards As(III) with detection limit 3.7 ppb.This type of material is also useful for rapid and precise As(III) detection in water samples [121].The analytical data of these material is quite acceptable however, the incorporation of toxic lead can be an obstacle in the way of commercialisation and encouragement for further research [121].

Ionic liquid-based sensors
Au NPs modified with ionic liquids such as tetradecyl(trihexyl)phosphonium chloride (Au NPs/phosphonium ionic liquid) as As(III) specific probe was reported as naked eye sensor.The level of As(III/V) below the optimum WHO guideline was made possible to be sensed, DL 7.5 μg/L (7.5 ppb).The effect of coexisting cations and anions was negligible and results were validated by laboratory based reliable technique, HPLC-AFS (AFS atomic fluorescence spectrometer).In addition to As(III/V) the same material was applied for sensing Hg(II) and Pb(II) ions and very good selectivity and sensitivity was achieved [122].Tetradecyl(trihexyl)phosphonium chloride ionic liquid (IL) was found selective binder to As(III).It acts as colorimetric probe with Au NPs and leads to naked-eye detection of the target ion below WHO threshold concentration.This material is highly tolerant to coexisting ions and can be applied for the quantification of As(III) ion [123].Metal oxides rich in oxygen vacancies are alternative for noble metal free materials.ZnFe 2 O 4 microspheres (ZF-Ms) were applied in slightly acidic medium (pH 6) for electrochemical determination of As(III) ion.The surface of ZF-Ms was wrapped with carbon and nitrogen (CN) rich nanodots and modified with IL benzimidazolium-1-acetate. The resulted material CN@ZF-Ms-IL showed ultralow DL 0.0006 ppb and sensitivity 41.08 μA ppb −1 .The material outperformed for As(III) and is the focus of future research to overcome the problem of economic hurdles [124].The Fe 3 O 4 -room temperature ionic liquids composite modified SPCE shows better performance than noble metals contained sensors.This type of sensors offer direct detection with DL 0.0008 ppb (8 � 10 À 4 ppb) and with excellent sensitivity, 4.91 μA ppb −1 [125].Linkers free approach towards synthesis of Fe 3 O 4 NPs decorated Au NPs is successfully used for the purpose of As(III) detection with relatively lower DL of 0.22 ppb.During this approach the particle size and metal-metal ratio was considered crucial towards better efficiency.The prepared material was embedded in IL and was used for modification of GCE.Several properties of material were combined in this type material such as good conductivity of IL, catalytic properties of Au NPs and high adsorption capacity of Fe 3 O 4 and the target ion was precisely detected with DL 0.22 ppb.Interference from other coexisting ions and humic acid in their 100-fold higher concentration was negligible [126].The sensing at Fe 3 O 4 NPs-Au NPs-IL interface is reliable and accurate platform for analysis of real environmental samples.
Ionic liquids are yet to be fully explored towards detection of toxic species present in water.Based on their lower detection limit to ppt level, they have very bright future in the field of sensors.

Biosensors for As(III/V) detection
Biomaterials are green in nature and do not create environmental issues due to their biodegradability and compatibility.There are numerous studies where biological molecules, have been coated on the surface of electrode for sensing applications.These modified electrodes have shown excellent performance in As(III) quantification below the WHO threshold limit.The fabrication process of biomaterial into a sensor is technical and its immobilisation and orientation on the sensor's surface is crucial [127,128].If fabricated in a technically correct way, then they are highly selective towards specific targets but they need strict conditions of pH and temperature etc., to achieve the required stability and reproducibility.Since the discovery of Fe 3 O 4 NPs to show peroxidase-like catalytic activity, nanozymes have attracted attention for applications in several fields including sensing As(III) and other toxicants [129].These artificial enzymes are robust, produced at low cost and can be used under harsh conditions [130].Nanozymes work under the principal of catalytic/enzymatic reactions under mild conditions.Biosensors are very effective but combination of portable spectrophotometer for As(III/V) in real water samples is needed to materialise the concept.Some of the sensors developed for real samples have been discussed for As(III) sensors here.Fluorescent active G-/T rich ssDNA QDs (quantum dots) synthesised at relatively low temperature are unique for colorimetric detection of As(III) ion.Detection was made possible due to arsenite-enhanced fluorescence of DNA QDs in the range 1-150 ppb (DL 0.2 ppb).Due to negligible interference of coexisting ions, biocompatibility and high selectivity of the sensor materials, are considered positive aspects towards arsenic detection [131].Aptasensors is a comparatively new field for sensing applications of heavy and other toxic elements and has attracted great scientific attention [132].The target ion As(III) interacts with aptamers to afford a cationic polymer complex (As(III)-aptamer complex) which efficiently aggregates Au NPs and a prominent change in colour is observed.The colorimetric determination of the target ion has been carried out with DL 5.3 ppb.The PDADMA (Poly(diallyldimethylammonium)), a water-soluble cationic polymer and Ars-3 aptamers were used for the purpose of As(III) detection.The resulted duplex cannot aggregate AuNPs, when As(III) is added it makes selectively a complex with Ars-3 aptamer and the PDADMA is released.The PDADMA aggregates Au NPs and ultimate colour of the solution changes which is measured calorimetrically.The intensity of colour is directly related to the concentration of As(III) ion.The aptamer/Au NPs material was tested for 14 cations including As(III) and As(V).The efficiency in the presence of these cations was selectively high for As(III) followed by As(V) [133].Silica nanoparticles coated with streptavidin (SNPs-streptavidin) make strong fluorescent material which could be used for sensing applications.It is simple and rapid with respect to response towards target ion.When As(III) is mixed with the modified NPs, certain changes take place which cause fluorescence signal.Each ion in water sample has its specific characteristics and in case of As(III) the normal interferants produced no effect on net results.The sensing materials showed a wide linear range 2-500 nM and a DL 0.45 nM (equivalent to 0.033 ppb) [134].Fabrication of AChE on SPCE, an inhibition based sensor for As(III) analysis of real water samples was guided by amperometric essay.The sensor possessed a dynamic linear range and sensitivity and stability of 150 days at normal temperature of 20-24°C.The detection limit of the sensor being recorded during experiments is reasonable, 150 ppb, it works on the basis of 4-acetylphenpl hydrolysis and is disposable after single use.It needs improvements to bring the detection limit below 10 ppb in future researches [135].A platinum electrode modified with ruthenium(II)-tris (bipyridyl), GO and AChE, the inhibitive response of the sensor in the presence of As(III) and Cd(II) was very good.For As(III) the detection limit is 0.03 μM (≈2.26 ppb), its sensitivity 106.05 μA μM −1 cm −2 , dynamic range of 0.05-0.8μM (94.2% inhibition) and a shorter response time 2 s are attractive properties.The sensing activity towards Cd(II) was almost the same as shown towards As(III).It was concluded that the Pt/Ru(II)-tris (bipy)-GO/AChE based sensor is superior in terms of sensitivity and linear range [136].Immobilisation of the Alcaligenis faecalis Bacteria on Au NPs-modified SPCE (AF/Au NPs-SPCE) was carried out for electrochemical sensing of As(III) ion.The detection is because of catalytic activity of arsenite oxidase enzyme.The As(III) is oxidised to As(V) and an analytical signal is produced.Low detection limit 6.61 μmolL −1 (6.61 ppb) in a solution of pH 7 and 1.84 μmol L −1 (1.84 ppb) in pH 12 was obtained for river water [137].The L-cysteine (Lcyst) and reduced lipoic acid (rLA) capped gold nanoparticle composite provides a sensing platform with an indirect band gap energy of 1.96 eV.The semiconducting material possesses good electrochemical properties.The surface modification with Lcyst and rLA prevents the NPs agglomeration and increase susceptibility of the sensor towards efficient bonding with cations of interest.The sensor (Lcyst/rLA/Au NPs/ SPCE) shows DL 3 ppb and relatively wider dynamic linear range 3-35 ppb [138].The recoveries in spiked samples during As(III) sensing was excellent but the interference of Cu(II) drastically hindered the sensing capacity towards As(III).L-tryptophan modified GCE as sensor (L-tryptophan/GCE) was active in detecting As(III) in water samples with detection limit of 12 pM (equivalent to 0.0009 ppb) [139].It indicates that there is a great potential in biosensors for on-site detection of trace amounts of As(III) in environmental samples.Citrate capped Au NPs interacts efficiently with As(III) which has been observed by colour change of the resulted solution (wine-red to blue).The lower detection limit of the sensor 1.8 ppb was determined spectroscopically.The techniques is simple as it does not need surface modification of nanoparticles and is applicable for determination of the target ion in drinking water samples [140].Effect of interfering cations was studied and was found negligible, slight interference was due to the existence of H 3 AsO 3 and H 2 ASO 3 − which forms a stable H-bond with citrate ions causing aggregation of the material.
Efficient and specific binding ligand to the surface is a key for development of a sensor.Peptides bearing high N-contents are efficient As(III) binders and they induce aggregations in Au NPs.Upon binding with As(III) the competitive binding between the target ion and peptide (T-Q-S-Y-K-H-G) prevents the aggregation thus leads to colorimetric detection of As(III) ion.The colorimetric assay is very sensitive and highly selective towards As(III) over other coexisting ions, DL was 54 nM (4 ppb) [141].Colorimetric determination of As(III) was also carried out through inhibition of reassembly-induced oxidase like activity inhibition by Pd-DTT (dithiothreitol-capped Pd NPs).Pd-DTT in the presence of oxygen triggers the colour of 3,3ʹ,5,5ʹ-tetrabethylbenzidine (TMB) from colourless to blue due to TMBox complex formation.In the presence of As(III), DTT makes a chelate through its sulfydryl group leading to reassembly of Pd-DTT.The reassembled material is weaker in activity and suppresses conversion of TMB in to TMBox.This colorimetric phenomenon leads to highly sensitive detection of As(III) in water samples.The essay was found to be very effective in a wide linear range, 33 ng L −1 to 333.3 μg L −1 and DL 35 ng L −1 (0.035 ppb) which is suitable for samples containing trace amounts of As(III) ion [142].The same strategy was used by Xu et al by double masking the interlayers active channels for oxidase-like Fe-Co-LDH (Fe-Co-layered double hydroxides).The high activity of Fe-Co-LDH nanozyme causes a TMBox complex with maximum absorbance at 652 nm.The active sites being masked by the As(III) are no more available due to firm interaction between two layers further supported by 3-MPA and As(III) ions, Fig S2 .The method is applicable through a wide linear range 0.10-8.33μM and DL 35 nM (2.63 ppb).This strategy is highly specific and potential interferants were found to have no adverse effects on As(III) sensing [143].Colorimetric determination of arsenite based on the analyte induced aggregation of citrate stabilised Au NPs with enhanced peroxidase-mimicking activity.Au NPs with mean size ca 20 nm exhibited negligible mimicking activity, by addition of As(III) rapid aggregation of Au NPs occurs through interaction between citrate stabiliser and the As(III) on NPs surface.This situation leads to accelerated peroxidase-mimicking activity of TMB.Based on this catalytic phenomenon colorimetric detection of As(III) was carried out with a wide linear range and DL below WHO guidelines (0.01-11.67 nM L −1 and 8 ppb, respectively) [144].
Aptasensors platform constructed for sensing of As(III) with signal amplification strategy by using various kinds of materials such as CNT-BSA (bovine serum albumin) works in an intelligent way.Detection limit of optical aptasensors has been reported as low as 1.3 � 10 À 3 and 9.3 � 10 À 6 nM for a variety of sensors in recent past [145][146][147].The same limit for available electrochemical optasensors has been reported in the range 1.9 � 10 À 6 nM [148].Signal amplification of Ars-3/Au NPs/SPCE by methylene blue (MB) and GO has led to detection limit as low as 2 � 10 À 4 mg L −1 .The electrode material is highly tolerant to interference, reproducible and sensitive towards As(III) ion [149].Au NPs functionalised with certain biologically compatible molecules have proved to be very efficient candidates in As(III) detection.Glutathione functionalised Au NPs (GHS-Au NPs) in colorimetric determination was able to detect As(III) well in the recommended concentration level.When As(III) and GHS-Au NPs are mixed together, a strong As-S bond between As(III) and GHS is formed which leads to Au NPs aggregation and a rapid colour change is observed.This type of material has shown an excellent DL 0.003 ppb [150] and 0.12 ppb [151], in different studies with negligible interference from coexisting cations.These sensing probes can be used for real time and on-site water analysis of As(III).GHS-Au NPs in another study shows dual mode detection, detection limit determined by colorimetry (0.11 ppb) was better than SERS (0.14 ppb) [152].Liquid crystals (LCs) based biosensors towards As(III) in water are also very sensitive within the desired limit.The aptamer Ars-3 firmly interacts with As(III) as discussed earlier which causes certain changes to govern the sensing behaviour.Cetyltrimethylammonium bromide (CTAB) is a cationic surfactant with a chemical formula [C 16 H 33 )N(CH 3 ) 3 ]Br and cause homeotropic orientation of LCs as a result of self-assembly of CTAB molecules.Ars-3 aptamer in the absence of As(III) causes a transition of orientation of LCs from homeotropic to planar.When As(III) is added, it makes an Aptamer-As(III) complex, resulting a conformational change in the aptamer, thus weakens the interaction between CTAB and aptamer which in turn leads to restore the actual orientation of LCs (Fig S3).The phenomenon was observed with the help of polarised light microscope and a detection limit towards As(III) was reported close to 3.7 ppb [153].A simple, selective and sensitive CeO 2 -DNA nanoprobe was also tested for As(V) with successful and precise detection in water samples [154].
So far, the lowest detection limit is reported for biosensors, L-tryptophan/GCE [139] and Ars-3/Au NPs/SPCE [149].These sensors have additional characteristics of high selectivity and excellent sensitivity.The effect of interference is least, several of them are colorimetric and are suitable for fast detection of metal ions.

Material with Dual Function
There are materials which show dual function they can be used for sensing and removal or can be used for simultaneous detection of more than one ions including As(III).Here few examples are discussed, with the intent to encourage future research towards multifunctional materials.Remediation of toxic elements is carried out by several technological ways: chemical, biological and physico-chemical etc.These remediation approaches generally deal with isolation, toxicity reduction, extraction or immobilisation [155][156][157][158][159]. Electrochemical sensing and removal is at the forefront of all technologies and to this end promising results have been obtained [160].In recent technological advancements, simultaneous detection and removal using the same material is underway.In terms of quantification of As(III) in drinking water samples, considerable materials have been introduced which give results within WHO recommended guidelines but several other characteristics including removal of this deadly toxic species has to be materialised in a cost effective way.In this context the peroxidase like activity of L-arginine modified FeOOH, 2Arg@FeOOH (discussed above for different material) has been utilised for detection as well as separation of As(V).This type of material is capable to detect As(V) in linear range 0.67-333.33μg L −1 and DL 0.42 μg L −1 (0.42 ppb).The same material was loaded on the surface of a fibre membrane and the modified surface was found 95% efficient in As(V) removal.The material has excellent reproducibility and possesses antibacterial activity too [161].A 3D flower-like Iron alkoxide have also been used for the same dual activity with colorimetric detection limit of 1.57 μg L −1 (1.57ppb) and the identical linear range as discussed above.The As(V) removal/adsorption efficiency of the material at 25°C was recorded to be 97.14 mg/g [162].Simultaneous determination of As(III) and Cu(II) was carried out at pH 9.5 with an electrode loaded with Au nano-stars.Results obtained with the help of SWASV gave DL 2.9 and 42.5 μgL −1 towards As(III) and Cu(II) ions, respectively.This type of sensors will prove very good for samples where the concentration of Cu(II) is high enough to cause interference in precise determination of As(III) ion.The results obtained with the help of this type of sensor were validated with graphite furnace atomic absorption spectroscopy (GF-AAS) for real samples of water and human blood serum.In this way the interference caused by Cu(II) during As(III) can easily be overcome [163].The presence of iodide provided by (KI) enhances the sensitivity of material because of its affinity to make a stable compound with Cu(II), thus suppressing its interfering effect [53].

Basic chemistry, challenges and possible solution
Polymeric material and nanoparticles functionalised with organic molecules enhances the sensing capacity of the material.Some organic molecules discussed in this article are shown in Fig S4, a critical look reveals that they are functional material and possess electron rich sites.These electron rich sites act as potential ligating points and efficiently coordinate to cations such as As(III) etc.Since As(III) is relatively soft in comparison to As(V) therefore, these ligands will even prove selective in sensing As(III) and As(V).The hard and soft acid base concept works well in addition to electrochemical processes.The chemistry involved during As(III) is usually determined by colour change (colorimetry), excitation followed by emission (fluorescence), redox phenomenon (electron process) or any other.During interpretation care shall be taken because these processes are always not straightforward [164].Pt electrodes cause catalytic oxidation of As(III) to As(V) during sensing process in a stepwise reaction, as given below.This redox phenomenon governs the concentration of As(III) ion.Metal nanoparticles cause redox phenomenon of As(III/0) on the surface of the material where three electrons are involved.Surface functionalization with compounds rich in donor sites as shown in Cu(II) is a major interferant during detection and quantification of As(III).The coexistence of Cu(II) cannot be avoided because it comes from natural sources and exists in large abundance.Its permissible limit according to WHO guidelines is 1.3 mgL −1 (1.3 ppm).During analysis of As(III) in water samples the results are greatly affected.Reproducibility of results with the same material under identical experimental conditions is always a challenge.The commercialisation of the senor cannot be materialised in many cases due to extensive interference by Cu(II) and several other interferants.It has been reported that a bimetallic alloy Cu 3 As 2 is formed on the surface of the electrode which inhibits further As(III) sensing thus leading to erroneous results.The interference caused by Cu(II) ion can easily be eliminated by a complexing reagents such as ammonia which is an efficient coordinating ligand with Cu(II).Addition of 7.8 M ammonia solution to water samples causes to precipitate out Cu(II) ion which can be separated via filtration followed by As(III) determination.This complexometric masking approach has been used for getting reliable results related to As(III) detection [98].EDTA also acts as hexadentate ligand and affords water-insoluble complex with Cu(II) ion and is therefore used as Cu(II) masking agent for avoiding its interference [74].Chemistry of thiosulphate is almost similar to that of Ammonia and EDTA in masking Cu(II) ion.This reagent S 2 O 2À 3 reacts in stepwise manner, Cu(II) is reduced to Cu(I) and a yellow insoluble complex Cu 2 S 2 O 3 is obtained which undergoes disproportionation reaction.In order to prevent disproportionation reaction, further one equivalent of thiosulphate ion reacts to give a stable complex which can easily be separated before experiment is run for As(III) detection.Thiosulphate ion can also be used for reduction of As(V) to As(III) ion.Electrochemical behaviour of As(III) and As(V) is different, to achieve better sensitivity and precise results, prior to reduction of As(V) to As(III) must be carried out [96].Thiosulphate can be used for its reducing ability towards As(V) or complexometric masking of Cu(II) to minimise interference towards As(III) determination [165].
Humic acid (HA) is used as agricultural supplement, it interferes with voltametric detection of As(III), through possible complexation with Au electrode which in turns causes weak complexation between HA and As(III) ion.When Fe(III) in addition to As(III) is added to the solution, the HA prefers to make stable complex with Fe(III) as compared with As(III) ion.When avoiding HA interference during detection of Au electrode, Fe(III) is better candidate to be used for better results [166].
Point-of-use (POU) sensors are of recent research interest, they are reliable and capable of fast detection.A radial microfluidic device working under integrated technology of ion concentration polarisation (ICP) and electroactive surface, is able to sense As(III) up to 1 ppb in arsenic affected fields' samples [167].Besides, single organic molecules as sensory devises are also in field of research with the intent to compete the nanotechnology.Schiff bases are very efficient ligands and have been tested for the purpose of As(III) sensing down to ppb level in real environmental samples.As(III) below 10 ppb was determined in a short time of ca. 10 s with no interference from coexisting ions [168].Although a considerable advancement in sensing heavy elements has been achieved, but still there are some serious issues which need to be resolved.
(1) Several sensors are associated with issues of reproducibility and signal stability especially at lower concentration of the target metal ion.(2) Homogeneity and controlled size distribution of nanostructures on large scale is a serious hurdle in the way of nanotechnology.(3) Water quality varies dramatically in different sampling locations along with parameters such as pH which makes it difficult to use the same machine throughout.(4) Fabrication of fully automated portable sensor for real-time analysis of heavy metals on a single platform is still a dream.(5) Evolution of Hydrogen gas occurs when performing experiment in acidic solutions which causes undesired corrosion of the electrode surface.Most of the sensors operate in neutral medium.

Conclusion
Electrochemical, colorimetric and fluorescent sensors containing different material like polymer nanocomposites, nanoparticles, metal nanoparticles, carbon material and ionic liquids are focus of current research.As discussed in this article, the most efficient materials are those containing ionic liquids and biopolymers.Limit of detection obtained for these two types of material is far lower as compared to WHO recommended limit and can be used for real water analysis.Monitoring of As(III) in drinking water with respect to limit of detection is no more a challenge, but other properties of sensors have to be improved for commercial and on-site determination.Material with ultrahigh antiinterferences activity and commercially viable need to be developed for efficient sensing.Replacement of noble metals or their ions with cheaper elements can reduce the cost of the sensor.Hopes are also associated with integrated technology such as microfluidic device working under ion concentration polarisation and electroactive surface.Multifunctional materials to detect As(III) followed by its effective removal is also a new and cost effective approach to be developed in future.Determination of As and other metal ions at ppt level will help avoiding catalyst poisoning in industrial processes.
Fig S4, can cause As(III) arrest at the surface of the modified electrode which causes certain electrochemical changes and thus the ion in water is detected (Fig S5).