FIELD OF THE INVENTION:
This invention relates to a method for the simultaneous electrochemical
determinaiion of inorganic contaminanss of water.
This invention further relates to an ultra sensitive, simultaneous
electrochemical determinaiion of inorganic contaminanss of water such
as arsenic, mercury and copper.
BACKGROUND OF THE INVENTION:
Arsenic (As) and mercury (Hg) are highly toxic and contaminaiion of
water by these toxic elements is a major problem in many countries.
Drinking water contaminated with As(III) and Hg(II) is associated with
number of diseases such as skin lesions, keratosis, lung cancer, bladder
cancer, kidney and respiratory failure, damage in the gastrointestinal
tract and nervous system, impairment of speech, hearing and working
etc. Contaminaiion of As(III) has been reported in various parts of the
world. Mercury is one of the heavy metals, highly toxic and harmful to
the environment and human health. World Health Organization (WHO)
has set the guidelines value 10 ppb for As(III). On the other hand, the US
environmental protection agency (EPA) has set the maxImum
contaminant level of mercury in drinking water at 2 ppb. Thus
determinaiion of trace level of As(III) and Hg(II) at the guideline value set
by WHO is of particular importance.

Various methods including hydride generation atomic fluorescence
spectrometry, inductively coupled plasma atomic emission spectrometry
(ICPAES)) inductively coupled plasma mass spectromerry (ICPMS),
fluorescence spectrophotometry, atomic absorpiion spectromerry etc.
have been used for the detection of As(III) and Hg(II). Although these
methods are successful in detecting As(III) and Hg(II) at sub-picogram to
sub-nanogram level, they require expensive instruments, laboratory set-
up and high operating cost. In contras,, the electrochemical methods are
highly sensitive and involve low cost equipments and laboratory set up.
The stripping voltammetric methods provide an efficient and reliable way
to detect arsenic and mercury at low concentraiion. Au coated diamond
and glassy carbon (GC) electrode and Au micro wire electrode have been
employed for the detection of As(III) and Hg(II). Nevertheless, the major
problems associated with the available electrochemical methods are (i)
the interference due to other metal ions like Cu(II) present in the natural
water (ii) high detection potential and (iii) interference due to supporting
electrolyte anions. The concentraiion of Cu(II) in natural/drinking water
is relatively high and it greatly interferes the measurement of As(III) due
to the formation of intermetallic compound such as CU3As2. Because of
the interference due to Cu(II) and other supporting electrolyte anions,
simultaneous determinaiion has not been achieved. Furthermore,
although As(III) and Hg(II) has been detected individually by

electrochemical methods, simultaneous detection without interference
from other coexisting ions has not been achieved.
For the electrochemical detection of As(III) and Hg(II), various solid
electrodes have been employed in the art. Recently the nano-sized metal
particle electrochemically deposited electrodes have been used for the
electroanalysis of As(III). Oai and Compton and co-workers (Anal. Chem.
2004, 76, 5924-5929, Anak Chem. 2004, 76, 5051-5055) have studied
the detection of arsentte using various electrodes. Ivadine et. al (Anal.
Chem. 2006, 78, 6291-6298) utilized iridium-implanted boron-doped
diamond (BOD) electrode for the detection of As(III) in ppb level. Very
recently, Song and Swain (Anal Chem. 2007, 79, 2412-2420) used the
Au-coated diamond thin film electrode for the voltammetric
determinaiion of As(III) and As(V). Luong and co-workers (Anal Chem.
2007, 79, 500-507) very recently reported the reusable Pt nanoparticle
modified BOD microelectrodes for the oxidative determination of As(III) .
The Au nanoparticle modified electrodes are known to be highly sensitive
in the electrochemical detection of As(III). The major problems associated
with the available electrochemical methods are (i) the interference due to
other metal ions like Cu(II) present in the natural water (ii) high detection
potential and (iii) interference due to supporting electrolyte anions. Cu(II)
forms intermetallic compound such us CU3As2 with As(III) and the

accurate measurement of As(III) has not been achieved due to this
interference.
In the case of Hg (II), unmodified and chemically modified electrodes have
been used for its detection. Microelectrode and microelectrode arrays
iridium and Au have been used for the detection of Hg(lI) (Anal. Chem
2006, 78,6291). Au microwire electrode has been recently used for the
detection of Hg(II) in seawater (Anal. Chem. 2006, 78, 5052-5060.. The
major concern with these electrodes is the lack of long term stability and
it requires medium exchange or surface regeneration.
PCT/GB06/03977 discloses electrochemical methods and materials for
the detection of arsenic. In one aspect, arsenic is detected using a
working electrode comprising particulate platinum. In another aspect,
arsenic is detected using an electrode comprising indium tin oxide and
particulate gold. Also provided are methods for the production of
electrodes which involve the electrodeposition of Au onto indium tin
oxide. The inventors have used the glassy carbon and indium tin oxide
(ITO) electrodes modified with Pt and Au nanoparticles for the detection
of As(III). As(III) has been detected on the Pt nanoparticle modified
electrode by its oxidation to As(VI) whereas the measurement has been
made on the Au nanoparticle by the oxidation of electrodepostted As(0))
The detection limit achieved is 2.1 and 5 ppb on Pt and Au nanoparticle

modified electrodes, respectively. The interference due to Cu(lI) on the Au
nanoparticle modified electrode has not been evaluated.
US Patent No. 5385708 entitled "Determination of ultra low levels of
mercury teaches a highly specific and sensitive electrode for the
determinaiion of ultra-low levels of mercury and an analytical system
based on such electrode. The electrode is a glassy carbon electrode spin-
coated with a monolayer of a highly sensitive reagent for the detection of
mercury. The analytical method based on the use of this type of electrode
is a voltammetric method. Concentraiions of the order of as low as about
2.1012 Moles mercury can be detected and measured. The reagent is
4,7,13,16,21,24-hexaoxa-l,10-diazabicyclo-[8.8.8] hexacosane for the
detection of Hg (II). This measurement is based on the complexation of
Hg(II) with the aforementioned complexing agent. The concentraiion of
Hg(II) has been monitored from the stripping curren.. Because this
method is based on the complexation, regeneraiion of the electrode
surface is critically necessary for repeated use.
US Patent No. 5391270 entitled 'Detection and measurement of heavy
metals' discloses an improved method for measuring the presence and
amount of a variety of metals contained in a sample. In the first step, all
of the various forms of each metal are converted to a soluble metallic
complex which is capable of being electrochemically reduced.

Voltammetry is then used to determine the stripping current or charge
characteristic of each metallic complex. Finally, the concentration of each
metal can be calculated by insertion of the stripping current or charge
value into an equation which correlates peak current or charge values
with metal concentration. The metals which can be detected and
quantified by using this method are gold, silver, bismuth, cadmium,
thallium, and mercury. The concentraiion of heavy metals is determined
using the stripping method. This method is based on the complexation of
the metal ions with iodide ion. The metal ions have been converted into
complexes of soluble form, which are electrochemically analyzed. The
interference due to other metal ions has not been addressed.
In view of the limitations of the prior art and the lack of a sensor that can
simultaneoully detect and determined inorganic contaminanss such as
Arsenic (III), Mercury (II) and Copper (II), the need exists in the industry
for such a system.
OBJECTS OF THE INVENTION:
It is an object of this invention to propose a sensor and a method, for the
simultaneous electro-chemical determinaiion of inorganic contaminanss
of water.

It is a further object of this invention to propose a sensor and a method,
for the simultaneous electro-chemical determination of inorganic
contaminants in which simultaneous determination is possible without
compromising the sensitivity.
Another object of this invention is to propose a sensor and a method, for
the simultaneous electro-chemical determination of inorganic
contaminants in which there is no interference from the major interferent
or surface active reagents in the electrochemical determination.
Yet another object of this invention is to propose a sensor and a method,
for the simultaneous electro-chemical determination of inorganic
contaminants of water, which shows wide linear response.
These and other objects and advantages of the invention will be apparent
from the ensuing description and illustrated with the help of the
accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 shows a diagrammatic representation of the scheme for the
fabrication of the sensor.

Fig 2A shows square wave anodic stripping voltammogram for the
detection of As(III) in 1 M HCI and the calibraiion plot.
Fig 2B shows square wave anodic stripping voltammogram for the
detection of Hg(II) in 1 M HCI and the calibration plot.
Fig 3 shows square wave anodic stripping voltammogram for As(III) (3.2
ppb) in the presence (b) of interfering Cu(II) (10 ppb) in 1 M HCl. (a) only
Cu(II).
Fig 4 shows square wave anodic stripping voltammogram for the
detection of As(III) in acidified real sample.
Fig 5 shows square wave anodic stripping voltammogram for the
simultaneous detection of As(III), Hg(II) and Cu(II) in 1 M HCl.
BRIEF DESCRIPTION OF THE INVENTION:
Thus according to this invention is provided a sensor for the
simultaneous electrochemical determination of inorganic contaminanss
of water.

According to this invention is further provided a method for the
simultaneous electrochemical determinaiion of inorganic contaminanss
of water.
In accordance with this invention is developed an ultra sensitive platform
based on particulates of gold (Au), for the simultaneous electrochemical
detection/determination of inorganic contaminanss As(III), Hg(II) and
Cu(II) without compromising the sensitivity.
The fabrication of this sensor involves the following procedures: The
conducting support is first modified with a layer of sol-gel 3-D silicate
network. This network has plenty of -SH functional groups. In this step
the conducting support i.e. the electrode is first modified with a thin
layer of silicate network derived from (3-mercaptopropyl)trimethoxysilane
(MPTS)) MPTS is known to form 3-D network structuee that is full of thiol
tail groups by the hydrolysis and condensaiion process. The MPTS sol is
prepared by dissolving MPTS, methanol and water (as 0.1 M HCI) in the
molar ratio of MPTS:methanol:water as 1:3:3 to 2:10:10 and stirring the
mixture vigorously for about 10 to 30 minutes. The conducting support
is an electrode having a surface selected from polyctystaliine gold,
coinage metal, platinum and palladium metal.

Gold (Au) nanopariiculate ensembles have been grown on the thiol
groups of the silicate network by colloidal chemical approach, The citrate
stabilized nanopariiculates are first self-assembled on the thiol groups of
the MPTS network modified electrode by chemisorpiion. The thiol group
has strong affinity to coinage, Pt and Pd metal surfaces and chemisorb
on the surface of the metal though the cleavage of S-H bond. The citrate
stabilized Au nanopariiculates are obtained by mixing trisodium citrate
in a mole percent of 1-2%, HAuCl4 1-2% and NaBH4 in a mole percent of
0.08-0.1% and stirring at room temperatuee in a mole percent of normal
pressure. The silicate network modified electrode is soaked in the as-
synthesized Au nanopariiculate for 12-18 hr at room temperature under
normal pressure.
The size of these nanopariiculates on the silicate network is enlarged by
seed mediated growth approach using hydroxyzine and HAuCl4.
Hydroxylamine is capable of reducing Au3+ to bulk metal and it has been
shown that this reaction is accelerated by Au surfaces. The surface-
catalyzed reduction of Au3+ by hydroxylamine leads to the enlargement of
the small particles on the network. Hydroxylamine is added in 0.01 to
5mM and HAuCl4 in 0.01-lmM. The mixture of hydroxylamine and
HAuCl4 is shaken constanlly at 200 to 500 rpm, keeping the electrode
inside the mixture. The size and morphology of the nanopariiculates on
the silicate network have been examined by FESEM and diffuse

reflectance spectral (DRS) measurements. The nanopariiculates are
randomly distributed throughout the silicate network on the electrode
surface and have the size distribuiion between 70-100 nm and an
average size of 85nm. The FESEM image of this electrode confirms the
existence of ensembles of Au-nanoparticles.
A diagrammaiic representation of the process is shown in Fig 1. The
sensor has been experimentally tested with commercial and real
samples.
The invention will now be explained in greater detail with the help of the
following non-limiting example.
Example:
(3-mercaptopropyl)trimethoxysilane (MPTS) sol was prepared by
dissolving MPTS, methanol and water (as 0.1 M HC1) in a molar ratio of
1:3:3 and stirring the mixture for 30 minutes. This MPTS sol is added to
a polycrystalline gold electrode, which forms a thin layer of a 3-D
network on the electrode.
Sodium citrate in 1 mole percent, chloroauric acid (HAuCl4) in 1 mole
percent and sodium borohydride in 0.08 mole percent were mixed and
the mixture stirred together at room temperatuee and normal pressure.

The silicate modified electrode is allowed to stand in this mixture for 12
hrs at room temperature under normal pressuee for complete
immobilization of the Au nanoparticulate ensembles on the thiol groups
of the MPTS network of the modified electrode. Hydroxylamine
hydrochloride (0.3mM) and HAuC14 (0.3 mM) were mixed and shaken
constanlly at 200 rpm, keeping the modified electrode inside the mixture
to lead to enlargement of the Au-particulates on the network to an
average size of 85nm. This is confirmed by the FESEM image of the
sensor.
The sensor thus fabricated is subjected to various experiments, to
ascertain the properties thereof.
Square wave anodic stripping voltammetry has been used for the
detection of the aforemeniioned inorganic contaminants. The
electrochemical measurements were performed with computer controlled
CHI643 electrochemical analyzer. Electrochemical cell consists of three
electrodes. Working: nanoparticulate modified electrode; auxiliary: Pt
wire; reference: AgjAgCl saturaeed with NaCl. Arsenic and mercury were
deposited at the potential of -0.35 V for 100 s by the reduction of As(III)
and Hg(II) in 1 M HCl. The anodic stripping (reoxidation of As(0) to As(III)
and Hg(0) to Hg(II)) of electrodepostted As(0) and Hg(0) was performed in
the potential range of -0.35 to 0.7 V at the following optimized

parameters; frequency: 40 Hz, amplitude: 20 mV and potential
incremen:: 4 mY. The simultaneous detection of As(III), Cu(lI) and Hg(lI)
has been performed at the same experimental condition as in the case of
As(III) and Hg(II).
The representative square wave anodic stripping voltammograms for
arsenic and mercury are shown in Figure 2 (A & B). The anodic peak
noticed at - 0.05 V in Figure 1a and - 0.5 V (Ag/AgCI) in Figure 1b
corresponds to the stripping of arsenic and mercury, respectively. It
should be mentioned here that the macrosized polycrystaliine Au
electrode does not show such response. The sensor is highly sensitive
and it shows linear response for As(III) and Hg(II)) The detection limit
(S/N=4) of the sensor towards As(III) and Hg(II) is 0.02 ppb, which is well
below the guideline value given by WHO. The sensitivity of the sensor
toward As(III) and Hg(lI) is 3.14 ± 0.01 and 2.95 ± 0.01 ^tA/ppb,
respectively. Selective detection is a challenging task with the real
sample, as the other ions commonly present in the groundwater can be
co-deposited and stripped off under the experimental condition used for
the detection of As(III) and Hg(II). Particularly Cu(lI) is a major interferent
in the detection of As(III). The electrodes that have high sensitivity toward
As(III) suffer from the interference due to Cu(II). However, the sensor
described herein does not suffer from the interference due to Cu(II). Two
distinct anodic peaks for As(IlI) and Cu(I1) were observed at 0.06 and

0.035 V, respectively (Figure 3). The stripping signal obtained for Cu(O) is
300 mV more positive than As (0). The stripping peak position and height
for Cu(O) does not change in the presence of As(III), confirming that the
sensor does not favor the formation of intermetallic compound (vide
supra). Most importantly, presence of Cu(II) does not affect the sensitivity
(3.17 ±0.01 pA/ppb) of the electrode toward As(III); no change in the
peak position for stripping of As(0) has been observed. The potential
application of the sensor for the detection of As(III) in real sample
collected from the arsenic contaminaeed water (24 North Parganas, West
Bengal) has been tested (Figure 4). The concentraiion of As(III) in the real
sample has been quantified using a standard calibration plot. Because
the voltammetric peak for the stripping of As(0)) Cu(O) and Hg(0) appears
at different potential with a separaiion of 180-300 mV between the
stripping peaks, the simultaneous measurement of all these three ions
has been achieved (Figure 5). The sensitivity of the electrode does not
change when As(III), Hg(II) and Cu(II) co-exist. Furthermore, this sensor
is free from the interference due to surface active compound exist in real
samples. This has been verified by measuring the concentraiion in the
presence of humic acid. Such simultaneous detection has not been
reported in the literature. The analytical performance of the sensor is
superior to the existing electrodes. Compared with the existing
electrodes, this sensor is ultra sensitive, selective, stable for a week and
can be used for the simultaneous measuremenss without compromising

the sensitivity. No change in the response has been observed during
continuous use for 24 hours.

WE CLAIM:
1. A sensor for the simultaneous detection of inorganic
contaminants, As (III), Hg (II) and eu (II) of water and having a
limit of detection of 0.02 ppb, comprising a conducting support
with a layer of three dimensional silica network with thiol tail
groups immobilized with citrate stabilized gold nanoparticles,
the gold particles having a size in the range of 70 to 100 nm.
2. The sensor as claimed in claim 1, wherein said conducting
support is an electrode having a surface selected from
polycrystalline gold, coinage metal, platinum and palladium
metal.
3. The sensor as claimed in claim 1, wherein the silicate network
is a three dimensional silicate derived from (3-
mercaptopropyl)trimethoxysilane(MPTS))
4. A process for manufacturing a sensor for the simultaneous
detection of inorganic contaminanss of water comprising the
steps of providing a conducting suppor,, adding a (3-
mercaptopropy)) trimethoxysilane (MPTS) sol thereto to form the
silicate network modified support, preparing a mixture

containing citrate stabilized gold nanopariiculates, soaking the
modified support in the mixture containing citrate stabilized
gold nanopariiculates to immobilize the Au nanopariiculates on
the silicate network of the modified suppor,, followed by
preparing a solution of hydroxylamine and chloroauric acid and
shaking the solution, keeping the electrode inside the solution,
to obtain the sensor.
5. The process as claimed in claim 4, wherein said conducting
support is an electrode having a surface selected from
po1ycrystalline gold, coinage metal, platinum and palladium.
6. The process as claimed in claim 4, wherein said MPTS sol is
prepared by dissolving MPTS, methanol and dilute hydrochloric
acid and stirring the mixture.
7. The process as claimed in claim 6, wherein said MPTS,
methanol and hydrochloric acid are present in a molar ratio of
1:3:3 to 2:10:10.
8. The process as claimed in claim 6, wherein a 0.1 M hydrochloric
acid is used.

9. The process as claimed in claim 6, wherein the mixture is
stirred for about 10 to 30 minutes.
10. The process as claimed In claim 4, wherein the Au
nanoparticles are citrate stabilized Au-nanoparticles.
11. The process as claimed in claim 4, wherein the citrate stabilized
Au nanoparticles are prepared by mixing trisodium citrate in 1-
2 mole percent, HAuCl4 in 1 to 2 mole percent and sodium
borohydride in 0.08 to 0.1%, at room temperature and normal
pressure.
12. The process as claimed in claim 4, wherein the modified
support is soaked in the mixture for 12 to 18 hrs at room
temperature and normal pressure.
13. The process as claimed in claim 4, for the solution of
hydroxylamine and chloroauric acid, 0.01-SmM of
hydroxyzine hydrochloride is used.
14. The process as claimed in claim 4, wherein for the solution of
hydroxylamine and chloroauric acid, 0.01 to 1 mM of
chloroauric acid (HAuCl4) is used.

15. The process as claimed in claim 4, wherein the solution of
hydroxyzine hydrochloride and HAuCl4 is shaken constantly
at 200 to 500 rpm.