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visual detection of Al3+ in ground water using ascorbic acid capped gold nanoparticles
Accepted Manuscript Title: Selective colorimetric/visual detection of Al3+ in ground water using ascorbic acid capped gold nanoparticles Authors: Lori Rastogi, K. Dash, A. Ballal PII: DOI: Reference:
S0925-4005(17)30563-4 http://dx.doi.org/doi:10.1016/j.snb.2017.03.138 SNB 22050
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
26-8-2016 16-3-2017 28-3-2017
Please cite this article as: Lori Rastogi, K.Dash, A.Ballal, Selective colorimetric/visual detection of Al3+ in ground water using ascorbic acid capped gold nanoparticles, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.03.138 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective colorimetric/visual detection of Al3+ in ground water using ascorbic acid capped gold nanoparticles Lori Rastogi1*, K. Dash1 and A. Ballal2 1
National Centre for Compositional Characterization of Materials, Bhabha Atomic Research Centre, ECIL-Post, Hyderabad - 500 062 Telangana, India
2
Molecular Biology Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085 Maharashtra, India
*Corresponding author: Lori Rastogi Email: [email protected] Tel: +91 4027131365 Fax: +91 40 27125463
Graphical abstract
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Research Highlights:
The colorimetric/visual method for the selective detection of Al3+ is developed. The AA@ AuNPs has been used for the sensitive detection of Al3+ in ground water. RIEX process has been used to improve sensitivity of the method in ground water. The LOD of developed method was found to be 12.5 ppb in ground water. The methods can be useful for on-field screening of water samples.
Abstract: A method for the selective and sensitive detection of aluminium (Al3+) in aqueous systems has been developed. The detection is achieved by the selective aggregation of ascorbic acid capped gold nanoparticles (AA-AuNPs) in the presence of aluminium which is observed by the change in colour of the colloidal solution from bright red to purple. The change in characteristic absorption peak can also be noticed spectrophotometrically; absorption of AAAuNPs (λmax – 520 nm) decreased and a second peak at 652 nm appeared after the addition of aluminium. The ratio of A652/A520 can be used to quantify the concentration of aluminium in water. The method gave a linear response from 20 ppb - 100 ppb (R2 = 0.996) of Al3+ in drinking water with a detection limit of 6.5 ppb. The proposed method did not suffer any interference from concomitant transition metal ions like: Mn2+, Ni2+, Zn2+, Sn2+, Li+, Co2+, Hg2+, Fe3+, and Pb2+ up to a concentration of 5 ppm and anions (Cl-, F-, SO42-, NO3, PO43-) up to a concentration of 250 ppm. However, a concentration of Ca2+ (≥ 15 ppm) was found to interfere with the detection of Al3+ in ground water. The interference was eliminated by passing the water through an anion exchange resin converted into oxalate form for the removal of the interferant as calcium oxalate precipitate in the resin phase. After this pre2
treatment the linearity range in ground water was found to be 100 - 350 ppb with R2 = 0.996 and LOD - 12.5 ppb. The simplicity and rapidity of the developed method shows great potential in favour of its application for screening of drinking water samples to check its safety with respect to aluminium toxicity. Key words: Aluminium detection, reactive ion exchanger (RIEX), gold nanoparticles, ascorbic acid, nano-sensor.
1. Introduction: Nanoparticles (NPs) based sensors are proven to be one of the most sought after sensing approach in various fields like: clinical diagnosis, forensic science, trace element analysis, environmental monitoring etc. Colorimetry [1-3], NIR imaging [4, 5], fluorescence quenching/enhancement [6, 7], surface enhanced Raman spectroscopy (SERS) [8, 9], and electrochemical sensing [10-12] are the various approaches which have been widely integrated with the development of nanoparticles based sensors. The most important considerations which have to be taken care while designing an effective nano-sensor is the higher sensitivity and selectivity at minimal costs and possibility to use them for on-field analysis. Taking these points into account, the most vital approach to design a nano-sensor is colorimetric approach, due to the key characteristic of noble metal nanoparticles; the localised surface plasma resonance (LSPR) [13]. Among other noble metal nanoparticles gold and silver NPs yields exceptionally high absorption coefficients [14] and scattering properties which results in having a higher sensitivity in optical detection methods than conventional organic dyes. This makes nanoparticles the preferred candidates for colorimetric sensing applications. Moreover, their LSPR properties can be easily modulated according to their 3
size, shape and composition [15]. Based on these properties gold nanoparticles (bare/functionalized) have been successfully explored as sensors for numerous analytes (antibiotics, pesticides, heavy metals, amino acids etc.) exploiting their change in LSPR maxima in aggregated and disaggregated state [16-27]. Aluminium occurs naturally in the form of silicates, cryolite, and bauxite rock. It comprises about 8.8% of the Earth’s surface. Aluminium has been identified in at least 596 of the 1,699 hazardous waste sites that have been proposed for inclusion on the Environment Protection Agency (EPA) National Priorities List (NPL) [28]. The exposure of human beings to aluminium is mostly occupational and by chronic use of aluminium-containing antacids, buffered aspirin and oral phosphate binders [29-31]. Food followed by drinking water is also the major intake source of aluminium [32]. It causes oxidative stress within brain tissue and has been related to Alzhimer’s disease [33-35]. Since the elimination half-life of aluminium from the human brain is 7 years, this can result in cumulative damage via the element's interference with neuro-filament axonal transport and neuro-filament assembly [36-38]. Thus aluminium contamination poses high risk to human health and environment hence the development of methods for its detection at low level is very crucial. Widely used approaches for analysing Al3+ includes: inductively coupled plasma mass spectrometry (ICP-MS) [39], inductively coupled plasma optical emission spectrometry (ICPOES) [40], atomic fluorescence spectroscopy (AFS) [41], and graphite furnace atomic absorption spectrometry (GF-AAS) [35, 42]. Although these methods offer high level of qualitative and quantitative analysis, but require a specialist for operation due to sophisticated instrumentation.
This also limits their on-field use which is very much required for
environmental monitoring. Recently several colorimetric methods based on the use of various organic reagents are reported for the determination of aluminium [43-46]. Some of these methods are less sensitive while others suffer from interferences and have limited 4
applications. As discussed previously nanoparticles based methods are recently being developed for the determination of aluminium. The recent reports which employed gold nanoparticles based colorimetric method for the determination of Al3+ either suffers from various interferences [47] or uses an elaborate method for the preparation of ligands and their functionalization on gold nanoparticles [48, 49]. In this paper, we report, a simple one step method for the preparation of ascorbic acid reduced/stabilized gold nanoparticles and their subsequent use for the detection and quantitation of Al3+. To the best of our knowledge AAAuNPs have not been reported for the detection of Al3+ till now. The proposed method is found to be highly selective for Al3+ and showed no major interferences from other concomitant metal ions up to ~ 5 ppm of concentration and anions up to 250 ppm. The method could be successfully used for the determination of Al3+ in ground water after passing through oxalate form of anion exchange resin, where excess calcium is removed by ionexchange precipitation as calcium oxalate in the resin phase. Thus in the present study,
ascorbic acid reduced/stabilized gold nanoparticles based
colorimetric method has been developed for the highly sensitive and selective detection of aluminium (Al3+) in ground water. As synthesized gold nanoparticles could be directly utilized for the sensing purpose without any further modification. The developed method is highly selective and suffers no interference from other transition metal ions. The only interference caused by Ca2+ at 15 ppm and above is also been corrected using reactive ion exchanger (REIX). The method holds great potential to be used as on-field monitoring of Al3+ for safe level screening in various aqueous matrices. 2. Materials and methods: 2.1 Reagents: Chloroauric acid trihydrate (HAuCl4 .3H2O) and AmberliteTM IRN 160 was obtained from Sigma-Aldrich, Bengaluru, India. Ascorbic acid and sodium hydroxide was obtained from 5
Merck, Mumbai, India. All the chemicals used are of analytical grade and used as supplied without any further purification. Standard stock solutions of Mn2+, Al3+, Ni2+, Zn2+, Sn2+, Li+, Co2+, Ca2+, Hg2+, Fe3+ and Pb2+ were prepared by dissolving appropriate amount of corresponding chloride and nitrate salts in de-ionized water with resistivity of 18MΩcm. 2.2 Preparation of gold nanoparticles: Ascorbic acid capped gold nanoparticles were synthesized using ascorbic acid as both reducing and stabilizing agent by a method proposed by Tyagi et al.[50] with some modifications. Briefly, 25 mL of 0.25 mM ascorbic acid was taken separately in three glass bottles and pH was adjusted to 5.0, 7.0 and 9.0 mL respectively. After that 450 µL of 10 mM HAuCl4 solution was added drop wise under stirring conditions. An immediate bright red colour appeared indicating the formation of gold nanoparticles. The final pH of synthesis was found to be 3.0, 3.5 and 4.0 respectively. The stability of synthesized AA-AuNPs at various pH (3.0, 3.5 and 4.0) was tested for a period of 30 days by recording absorbance at λmax at regular intervals. 2.3 Characterization of gold nanoparticles: The synthesized nanoparticles were characterized in order to investigate their size, shape and functional group present on the surface. Synergy H1 Hybrid reader (Biotek, Germany) was used to record surface plasma resonance (SPR) spectrum of the synthesized gold colloidal solution. The size and shape measurements were carried out using (Zeiss-Carl-Libra-120) transmission electron microscopy (TEM). The sample for TEM analysis was prepared by drop coating gold nanoparticles solution on a carbon coated copper grid. The size distribution graph was plotted by counting ~ 200 particles. 2.4 Assay for detection and determination of Al3+: The as synthesized nanoparticles were used for the colorimetric detection of Al3+ without any further purification. In order to demonstrate the detection of Al3+, 150 µL of the gold 6
nanoparticles were added to 50 µL of water containing different concentrations of Al3+ ions in a polystyrene 96-well plate and mixed immediately by gently shaking the solution. The UVvisible spectrum was recorded in order to quantify the Al3+ ion concentration using a calibration graph. The calibration graph was obtained by plotting known concentrations of Al3+ ions (0 – 100 ppb) against absorbance ratio A652/A520 nm. The selectivity of the proposed Au NPs preparation for Al3+ was confirmed adding various metal ions Mn2+, Al3+, Ni2+, Zn2+, Sn2+, Li+,Co2+, Ca2+, Hg2+, Fe3+ and Pb2+ followed by analysis using UV-visible spectrophotometer.
2.5 Preparation of oxalate form of anion exchange resin: In order to prepare the oxalate form of anion exchange resin (mean size < 0.300 mm), the microporus resin (AmberliteTM IRN160) was first regenerated by sequentially washing (three times of bed volume) with isopropanol, 1M HCl, 1M NaOH and finally activated with excess amount of sodium oxalate solution. The resin was then washed with de-ionized water till the washings were free from oxalate (checked by Ion Chromatograph). Dionex-ICS-3000 (Sunnyvale, CA, USA) with IonPac-AS-20. Before use 0.5 g of dry resin was transferred into syringe (2 mL capacity, Dispovan, India) to prepare a mini bed. Excess water from the resin was removed under pressure exerted by pressing the piston of the syringe. Resin thus obtained after removal of water is suitable for the column mode of operation, as the volume change due to uptake or release of solvent from the resin was minimal. 2.6 Application of assay for determination of Al3+ in drinking water and ground water: The Al3+ spiked drinking water and ground water samples were used to test the suitability of the assay for Al3+ determination in these matrixes. The water samples were first passed through syringe containing oxalate form of anion exchange resin, before testing for Al3+. In the filtered water samples Al3+ was determined as described the experimental section. 7
2.7 Statistical analysis: Three independent experiments were carried out and the results are represented as mean ± SD. The Student’s t-test was applied to calculate the statistical significance of the experimental data. 3. Results and discussions: 3.1 Synthesis and characterization of gold nanoparticles: Figure 1 represents the results of synthesis and characterization of AA-AuNPs. The red colour appeared within seconds after addition of HAuCl4 to a diluted solution of ascorbic acid. The UV-vis. spectrums of AA-AuNPs synthesized at various pHs (3.0, 3.5 and 4.0) are in shown in Fig. 1a. Appearance of bright red colour and a peak at ~ 520 nm in UV-visible spectrum clearly represents the synthesis of gold nanoparticles. This was further confirmed by TEM analysis; the micrograph shows nearly- monodispersed, spherical nanoparticles (Fig. 1b) of average size of 11.2 ± 4.2 nm (Fig. 1c). Since the nanoparticles prepared with 0.25mM of ascorbic acid at pH-4 provided the most sensitive response (section 3.2), the size analysis of this preparation was carried out by TEM. The AA-AuNPs when synthesized at higher pHs were blue to purple in colour and agglomerated within few hours of synthesis. Hence, the AA-AuNPs synthesized at pHs 3.0, 3.5 and 4.0 were chosen for further investigations. The concentration of reductant and pH of the solution is known to affect the size of the nanoparticles. The stability assessment of the AA-AuNPs synthesized at pH 3.0, 3.5 and 4.0 was carried out by measuring absorbance at 520 nm for a period of 30 days at room temperature since the stability of the colloidal preparation is very important if it has to be used for real time application. The results are presented in Fig. 1d; the graph shows that the synthesized AA-AuNPs remained stable for the tested period without any significant change in absorbance value. 3.2 Assay for detection and determination of Al3+: 8
The as synthesized AA-AuNPs were used directly for the detection of Al3+ without any further purification. In order to demonstrate the assay, 50 µL of water containing Al3+ ions from stock (5 ppm) solution was added to 150 µL of the AA-AuNPs solution in polystyrene microtiter plate wells and mixed by gentle shaking. A change in colour from red to purple was observed almost immediately indicating the aggregation of AA-AuNPs in the presence of Al3+. The UV-visible spectrum of control (without Al3+) and test samples (with Al3+) AAAuNPs were recorded to observe the change in characteristic spectrum of gold nanoparticles. The analyte based aggregation of gold nanoparticles is well established and have been widely used for the selective sensing. The Al3+ based aggregation of AA-AuNPs results mainly due to the interaction of ascorbic acid and Al3+ ions which is possibly more preferred than other metal ions at lower (ppb) concentration. The UV-visible spectrum shows the change of absorbance peak intensity at 520 nm and the appearance of a new peak at 652 nm in the presence of Al3+ (Fig. 2). These kinds of changes in UV-visible spectrum are being associated with the aggregation of gold nanoparticles [22, 51, 52]. A concentration of 0.25 mM of ascorbic acid was used for the synthesis of AuNPs because the AuNPs synthesized at 0.25mM gave the most sensitive response to Al3+. While the AAAuNPs synthesized at 0.1 mM concentration were not stable, and those synthesized at higher concentrations did not improve the sensitivity of the assay. It is important to mention that the detection of aluminium by AA-AuNPs was found to be highly pH dependent. The sensitivity of the method improved with increase in pH from 3.0 – 4.0. As represented in Fig. 3(a ,b) the visual response of AA-AuNPs to aluminium starts at 25, 50 and 75 ppb for the pH – 4.0, 3.5 and 3.0 respectively. Thus a nanoparticles preparation with final pH of 4.0 was selected for further investigations. This fact is further supported by the stability of de-hydro ascorbic acid (DHAA) at pH-4 which has been reported in earlier publications [53, 54].The significant role of DHAA for the formation of chelating complex has been described in detail in the section 9
3.3. The calibration graph was obtained by plotting known concentrations of Al3+ ions (0 – 100 ppb) against absorbance ratio of A652/A520 nm. A linear response was obtained from 20 – 100 ppb of Al3+ with R2 = 0.996. The limit of detection (LOD = 3σ) in deionized water was calculated to be ~ 1.3 ppb. The selectivity of the proposed AA-AuNPs based assay for detection of Al3+ ions was confirmed adding various other transition metal ions Mn2+, Al3+, Ni2+, Zn2+, Sn2+, Li+,Co2+, Ca2+, Hg2+, Fe3+ and Pb2+ followed by analysis using UV-visible spectrophotometer. As it is seen in Fig. 4 that except for Al3+ no other metal ions have shown significant decrease in absorption peak, however in case of iron there was a little increase in the peak intensity when measured by spectrophotometer but visually there is hardly any change in the colour of AuNPs in the presence of Fe2+ as seen in the picture incorporated in the Fig 4. 3.3 Mechanism of selective detection of Al3+ using AA-AuNPs: As shown in the given scheme 1 that the ascorbic acid mediated reduction of gold nanoparticles occurs via de-protonation of ascorbic acid in acidic media (pH 3 - 4) the electrons released in the process reduce Au ions to form gold nanoparticle; these nanoparticles are then capped by de-hydro ascorbic acid (DHAA). The resulting di-ketone form DHAA acts as a strong chelating agent for Al3+ and form oxo-complexes [55], which leads to selective aggregation of gold nanoparticles in the presence of Al3+ [Scheme 2]. Hence, a single reagent i.e. ascorbic acid works as a reductant and a stabilizer for AuNPs and later as a chelator for Al3+. The evidences of in vivo chelating effects of ascorbic acid against aluminium have been reported in the literature, for protecting against Al toxicity. In addition the effect of ascorbic acid on the aluminium absorption in the mice systems has been studied showing that in the presence of ascorbic acid the aluminium absorption was low [56-59]. These studies further suggest strong interaction of ascorbic acid and aluminium. 10
3.4 Application of assay for the determination of Al3+ in drinking water and ground water samples: The developed assay was explored for the determination of Al3+ in the drinking water after spiking with various concentrations (0-100 ppb) of Al3+. Figure 5 shows that the assay worked extremely well and a linear response was observed in the range of 20 ppb - 100 ppb for de-ionized water with R2 = 0.996. In order to further investigate the suitability of the assay in other aqueous matrices the assay was further tested for ground water samples. When this method was applied for the visual determination of Al3+ in different ground water samples, it was observed that in most of the cases, the sensitivity was drastically reduced. Only at concentration levels above 600 ppb level of Al3+, there was response by AA-AuNPs. However, as the maximum permissible limit of Al3+ is ~ 200 ng/mL, it was imperative to know which cations and/or anions normally present in ground water contribute/s to the observed degradation of sensitivity of the visual nano particle reagent. The selectivity of the assay against other transition metal ions has already been established and discussed in previous section. So we, further investigated the interference due to alkali metals, alkaline earth metals and anions present in ground water. In the Deccan region of India, ground water samples were normally found to contain Na+ (164 ± 117 mg/L) K+ (4.6 ± 3.4 mg/L), Ca2+ (110 ± 52 mg/L), Mg2+ (42 ± 21 mg/L) and anions Cl- (186 ± 104 mg/L), F- (1.35± 0.06 mg/L), SO42- (98 ± 68 mg/L), NO3- (99 ± 75 mg/L) [60]. These cations and anions were spiked in to DI water and the degrees of interferences were measured quantitatively using the spectrophotometer. It was observed that among the alkaline earth metals only calcium ion led to significant change in absorption spectrum while other cations did not interfere ( Fig. 6 a). The anions did not affect the absorption characteristic of AA-AuNPs even at the highest concentration tested (supplementary information Fig. 1). Thereafter, efforts were made to 11
investigate the levels at which calcium interference sets in. It was found that the interference begins at Ca2+ concentration ≥15 µg/mL, which could be observed even visually as shown in Fig. 6 b. Other cations and anions did not interfere even when they are present at the maximum level normally present in the ground water.
So in order to remove Ca2+
interference, different anions which are known to form insoluble precipitates with calcium were investigated. Calcium forms insoluble salts with anions like hydrogen phosphate, sulphate and oxalate as the corresponding salts have low solubility products (Ksp) of 2.07 x 10-7, 5.0 x 10-5 and 5.0 x 10-9 respectively. So in order to remove maximum calcium from the ground water, oxalate precipitation was preferred. However, by adding 300 ppm of oxalate while the interference was reduced, the visual method could only produce a colour change at and above aluminium concentration of 200 ppb. Further, the concentration of free oxalate ion present after precipitation of oxalate introduced additional interferences. At a concentration of 150 ppm of oxalate, the interference appeared, and at 600 ppm, the system failed to respond the visual test. These type of experimental observations indicated that a method which offered removal of calcium without introducing any appreciable concentration of oxalate into the test water sample would be preferable. Though, cation exchangers in H+ form could be used to remove Ca2+, such an approach will remove Al3+ due to the higher selectivity of the later towards the cation exchanger. Selective separation and /or preconcentration of cations and anions have been carried out by in situ precipitation on columns packed with ion exchange resins carrying suitable counter ions that would form sparingly soluble salts of the target species in the resin phase. Such selective separation or preconcentration is known as “reactive ion-exchange” (RIEX) [61, 62]. In the present case to separate calcium, oxalate was selected as counter ion of the anion exchange resin as the resulting calcium oxalate precipitates in the resin phase due to its low
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solubility product. The selective separation of calcium from the water sample passed in the column bed containing anion exchanger (oxalate form) can be represented by: (Rs NR3+)2 (oxalate) + Ca2+X2 (water)→ (Rs NR3+)2 X2 + Ca Oxalate (↓)Rs , Where, Rs = solid resin phase; X, is the anion of the Ca2+ cation, and Ca Oxalate (↓) = the precipitated calcium oxalate. 3.5 Assessment of the performance of the RIEX process towards Ca2+ removal Normally, ground water contains calcium between 70-200 ppm. When 1 mL of the water containing 200 ppm Ca2+ of was passed into the syringe containing the oxalate form of ion exchanger and the obtained water was analysed for calcium by ICP-AES; it was found that the Ca2+ concentration was reduced to 5-7 ppm, table 1(supplementary information), at which the visual method does not experience any interference. As the oxalate form of resin was washed with sufficient water prior to use, so leaching of the oxalate was not expected. However, after precipitation of the calcium oxalate, which is sparingly soluble, the oxalate ion concentration was also checked by ion chromatography. The oxalate concentration in the water was around 4-8 ppm. Thus both the calcium and oxalate concentrations were much below the concentration at which visual interference takes place for the gold nanoparticle. After passing the ground water through syringe containing the oxalate form of ion exchanger it was again tested for its response towards Al3+, the results are being plotted in figure 7. The linear response was observed between 100 - 350 ppb of Al3+ with R2 = 0.996. 3.6 Cross validation of developed assay using ICPMS; accuracy and precision: The developed AA-AuNPs based assay for the detection of Al3+ was further cross validated by comparing the results of the assay to the results obtained by ICPMS. The results are being compared in table 1. As evident from the table that the ground water contained only 10 ppb of Al3+ (ICPMS result) so it was not detected by AA-AuNPs. However when 100 ppb of Al3+ was spiked in the water, the response was clear and the obtained values were in concord with 13
the ICPMS at this level. The precision of Al3+ determination by AA-AuNPs assay in terms of relative standard deviation (RSD) was found to be 3 – 6% (intra-day) whereas as inter-day precision varied between 5 - 8%. As no certified material of Al3+ in ground water was available in our laboratory, the values obtained were compared with ICPMS results. The mean values obtained by both the methods were the subjected to statistical analysis for the comparison. In the two-tailed t-test, pooled standard deviation with degree of freedom (F = 4) was used to find out the calculated t value which was found to be 3.7. Thus, the calculated tvalue (3.7) was found to be less than the critical tabulated t-value of 4.6 (F= 4, p = 0.01). Hence there is no statistically significant difference between the mean obtained by AAAuNPs method and ICPMS results at 99% confidence interval. As discussed in previous section where the application of the developed method in drinking and ground water was tested that the linear range in ground water is 100-350 ppb with LOD of 12.5 ppb thus the method even though is not able to detect very low concentrations of Al3+ as could be done by ICPMS, it can still prove to be highly beneficial for the on-field screening of Al3+ in drinking and ground water systems for the on field screening even though it includes a sample pretreatment step. Since, the step is very simple where the water has to be passed through the REIX resin packed in a syringe, the resultant water can be directly used for the analysis. RIEX is oxalate form of anion exchanger and is prepared in the laboratory and can accompany the detection kit. At the time of test the ground water is passed through RIEX syringe and the treated water (50µL) is mixed with 150 µL of AuNPs, where the colour formation is instantaneous. So the time taken for the measurement is ~ 2 min. Further oxalate resin could be regenerated and reused. On-field and screening of large numbers of water samples using ICPMS is costly and time consuming. 4. Conclusions:
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In conclusion, the report demonstrated a simple, cost-effective colorimetric assay based on AA-AuNPs for the detection of Al3+. The developed method is successfully applied for sensing of residual Al3+ in various aqueous matrices. The sensitivity of method was found to be dependent on pH of the AA-AuNPs preparation, and the nanoparticles synthesized at pH 4.0 were found to be most sensitive. The method was found to be highly selective for Al3+ this was evident due to absence of aggregation of AA-AuNPs by any other alkali metals, alkaline earth metal and transition metal ions and anions tested. The only interference caused by Ca2+ at ≥ 15 ppm concentration could easily be removed by passing the water through REIX resin. The developed method can find enormous application in selective quantification of Al3+ in routine laboratory analysis or rapid on-field assay.
Acknowledgement: The authors would like to thank Dr. Sunil Jai Kumar, Head, NCCCM/BARC, and Dr. D. Karunasagar, Head, EACS, NCCCM/BARC, for his constant support and encouragement.
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Lori Rastogi obtained her M.Sc. from the Department of Biotechnology, University of Pune, Maharashtra, India. Presently she is working as scientific officer in National Center for Compositional Characterisation of Materials/Bhabha Atomic Research Center, Hyderabad, India. Her present research interests are development of facile and ecofriendly methods for the size and shape controlled synthesis of metallic (Ag, Au, Cu and Pd) nanoparticles and their application for the detection of environmental toxicants like heavy metals and organic pollutants. Dr. A. Ballal (Ph. D., Molecular Biology) heads the Biological Damage and Repair Section in the Molecular Biology Division, BARC and is affiliated to HBNI as an Associate Professor. He was a Post Doctoral Fellow, Univ. of South Dakota, USA, working on gene regulation in Staphylococcus. Dr. Ballal was awarded the Young Scientist Award (2006) by the Department of Atomic Energy. He received the Homi Bhabha Medal for securing the 1st rank in the Graduate Programme (1997) of the BARC Training School. His research is focused on deciphering mechanisms responsible for overcoming abiotic stresses and he is an avid electron microscopist. Dr. K. Dash obtained his Ph.D. from Osmania University and works as a senior scientist in National Center for Compositional Characterisation of Materials (NCCCM), Hyderabad under Bhabha Atomic Research Center (BARC), Mumbai, India. His research interest is analysis of high purity materials using spectroscopic and chromatographic techniques (ICP-AES/MS, HPLC, IC and GC-MS) and development of novel sample preparation steps using ion-exchange, vapour phase digestion and classical analytical chemistry. He has 35 publications in various international journals.
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Figure legends:
Figure 1: (a) UV-visible spectrum of AA-AuNPs synthesized at various pH, Inset: Digit photograph of colloidal solution, (b) TEM micrograph, and (c) a bar graph showing size distribution of AA-AuNPs synthesized at pH – 4.0, (d) the time (days) versus absorbance graph of AA-AuNPs synthesized at various pH at room temperature for a period of 30 days.
25
Figure 2: UV-visible absorption spectrum of AA-Au NPs solution as a response to Al3+. Inset: Visual colour change as a response of addition of Al3+ to AA-Au NPs solution.
26
Figure 3: Response curve of AA-Au NPs to Al3+ as a function of pH. (a) Ratio of A652/ A 520 vs. wavelength has been plotted, (b) Digital photograph of the AA-Au NPs after addition of Al3+ at different pH.
27
Figure 4: UV-visible spectrum of AA-Au NPs showing its selectivity against other transition metal ions.
28
Figure 5: A plot between ratios at A652/ A 520 versus various spiked concentrations (0 – 100 ppb) of Al3+ in drinking water.
29
Figure 6: (a) Absorbance spectrum of AA-Au NPs after addition of alkaline earth metals, (b) photograph showing the Ca2+ interference at various concentrations (0- 25 ppm).
30
Figure 7: (a) UV-visible spectrum of AA-Au NPs as a response to Al3+ concentrations in ground water matrix after pre-treating water with RIEX process. (b) Plot between ratios at A652/ A 520 versus various spiked concentrations (100 – 350 ppb) of Al3+ in ground water.
31
Scheme 1: Schematic representation of AA-AuNPs aggregation in the presence of Al3+.
Scheme 2: Mechanism for the aggregation of AA-AuNPs via formation of oxo-complex between Al3+ and dehydroascorbic acid [55].
32
Table 1: Comparison of results obtained by ICPMS and AA-AuNPs
Sample ID G.W.1 G.W.2 G.W.1 spiked with 100 ppb Al3+ G.W.1 spiked with 200 ppb Al3+
Concentration of Al3+ (ppb)* ICPMS AA-AuNPs 10.0 ± 1.5 -13.5 ± 0.7 -114.0 ± 1. 8 124.3 ± 3.6 207.5 ± 4.2
204.7 ± 5.9
* (Analysis based on three independent samples, n = 3: Values: Mean S.D.
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S0925-4005(17)30563-4 http://dx.doi.org/doi:10.1016/j.snb.2017.03.138 SNB 22050
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
26-8-2016 16-3-2017 28-3-2017
Please cite this article as: Lori Rastogi, K.Dash, A.Ballal, Selective colorimetric/visual detection of Al3+ in ground water using ascorbic acid capped gold nanoparticles, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.03.138 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Selective colorimetric/visual detection of Al3+ in ground water using ascorbic acid capped gold nanoparticles Lori Rastogi1*, K. Dash1 and A. Ballal2 1
National Centre for Compositional Characterization of Materials, Bhabha Atomic Research Centre, ECIL-Post, Hyderabad - 500 062 Telangana, India
2
Molecular Biology Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085 Maharashtra, India
*Corresponding author: Lori Rastogi Email: [email protected] Tel: +91 4027131365 Fax: +91 40 27125463
Graphical abstract
1
Research Highlights:
The colorimetric/visual method for the selective detection of Al3+ is developed. The AA@ AuNPs has been used for the sensitive detection of Al3+ in ground water. RIEX process has been used to improve sensitivity of the method in ground water. The LOD of developed method was found to be 12.5 ppb in ground water. The methods can be useful for on-field screening of water samples.
Abstract: A method for the selective and sensitive detection of aluminium (Al3+) in aqueous systems has been developed. The detection is achieved by the selective aggregation of ascorbic acid capped gold nanoparticles (AA-AuNPs) in the presence of aluminium which is observed by the change in colour of the colloidal solution from bright red to purple. The change in characteristic absorption peak can also be noticed spectrophotometrically; absorption of AAAuNPs (λmax – 520 nm) decreased and a second peak at 652 nm appeared after the addition of aluminium. The ratio of A652/A520 can be used to quantify the concentration of aluminium in water. The method gave a linear response from 20 ppb - 100 ppb (R2 = 0.996) of Al3+ in drinking water with a detection limit of 6.5 ppb. The proposed method did not suffer any interference from concomitant transition metal ions like: Mn2+, Ni2+, Zn2+, Sn2+, Li+, Co2+, Hg2+, Fe3+, and Pb2+ up to a concentration of 5 ppm and anions (Cl-, F-, SO42-, NO3, PO43-) up to a concentration of 250 ppm. However, a concentration of Ca2+ (≥ 15 ppm) was found to interfere with the detection of Al3+ in ground water. The interference was eliminated by passing the water through an anion exchange resin converted into oxalate form for the removal of the interferant as calcium oxalate precipitate in the resin phase. After this pre2
treatment the linearity range in ground water was found to be 100 - 350 ppb with R2 = 0.996 and LOD - 12.5 ppb. The simplicity and rapidity of the developed method shows great potential in favour of its application for screening of drinking water samples to check its safety with respect to aluminium toxicity. Key words: Aluminium detection, reactive ion exchanger (RIEX), gold nanoparticles, ascorbic acid, nano-sensor.
1. Introduction: Nanoparticles (NPs) based sensors are proven to be one of the most sought after sensing approach in various fields like: clinical diagnosis, forensic science, trace element analysis, environmental monitoring etc. Colorimetry [1-3], NIR imaging [4, 5], fluorescence quenching/enhancement [6, 7], surface enhanced Raman spectroscopy (SERS) [8, 9], and electrochemical sensing [10-12] are the various approaches which have been widely integrated with the development of nanoparticles based sensors. The most important considerations which have to be taken care while designing an effective nano-sensor is the higher sensitivity and selectivity at minimal costs and possibility to use them for on-field analysis. Taking these points into account, the most vital approach to design a nano-sensor is colorimetric approach, due to the key characteristic of noble metal nanoparticles; the localised surface plasma resonance (LSPR) [13]. Among other noble metal nanoparticles gold and silver NPs yields exceptionally high absorption coefficients [14] and scattering properties which results in having a higher sensitivity in optical detection methods than conventional organic dyes. This makes nanoparticles the preferred candidates for colorimetric sensing applications. Moreover, their LSPR properties can be easily modulated according to their 3
size, shape and composition [15]. Based on these properties gold nanoparticles (bare/functionalized) have been successfully explored as sensors for numerous analytes (antibiotics, pesticides, heavy metals, amino acids etc.) exploiting their change in LSPR maxima in aggregated and disaggregated state [16-27]. Aluminium occurs naturally in the form of silicates, cryolite, and bauxite rock. It comprises about 8.8% of the Earth’s surface. Aluminium has been identified in at least 596 of the 1,699 hazardous waste sites that have been proposed for inclusion on the Environment Protection Agency (EPA) National Priorities List (NPL) [28]. The exposure of human beings to aluminium is mostly occupational and by chronic use of aluminium-containing antacids, buffered aspirin and oral phosphate binders [29-31]. Food followed by drinking water is also the major intake source of aluminium [32]. It causes oxidative stress within brain tissue and has been related to Alzhimer’s disease [33-35]. Since the elimination half-life of aluminium from the human brain is 7 years, this can result in cumulative damage via the element's interference with neuro-filament axonal transport and neuro-filament assembly [36-38]. Thus aluminium contamination poses high risk to human health and environment hence the development of methods for its detection at low level is very crucial. Widely used approaches for analysing Al3+ includes: inductively coupled plasma mass spectrometry (ICP-MS) [39], inductively coupled plasma optical emission spectrometry (ICPOES) [40], atomic fluorescence spectroscopy (AFS) [41], and graphite furnace atomic absorption spectrometry (GF-AAS) [35, 42]. Although these methods offer high level of qualitative and quantitative analysis, but require a specialist for operation due to sophisticated instrumentation.
This also limits their on-field use which is very much required for
environmental monitoring. Recently several colorimetric methods based on the use of various organic reagents are reported for the determination of aluminium [43-46]. Some of these methods are less sensitive while others suffer from interferences and have limited 4
applications. As discussed previously nanoparticles based methods are recently being developed for the determination of aluminium. The recent reports which employed gold nanoparticles based colorimetric method for the determination of Al3+ either suffers from various interferences [47] or uses an elaborate method for the preparation of ligands and their functionalization on gold nanoparticles [48, 49]. In this paper, we report, a simple one step method for the preparation of ascorbic acid reduced/stabilized gold nanoparticles and their subsequent use for the detection and quantitation of Al3+. To the best of our knowledge AAAuNPs have not been reported for the detection of Al3+ till now. The proposed method is found to be highly selective for Al3+ and showed no major interferences from other concomitant metal ions up to ~ 5 ppm of concentration and anions up to 250 ppm. The method could be successfully used for the determination of Al3+ in ground water after passing through oxalate form of anion exchange resin, where excess calcium is removed by ionexchange precipitation as calcium oxalate in the resin phase. Thus in the present study,
ascorbic acid reduced/stabilized gold nanoparticles based
colorimetric method has been developed for the highly sensitive and selective detection of aluminium (Al3+) in ground water. As synthesized gold nanoparticles could be directly utilized for the sensing purpose without any further modification. The developed method is highly selective and suffers no interference from other transition metal ions. The only interference caused by Ca2+ at 15 ppm and above is also been corrected using reactive ion exchanger (REIX). The method holds great potential to be used as on-field monitoring of Al3+ for safe level screening in various aqueous matrices. 2. Materials and methods: 2.1 Reagents: Chloroauric acid trihydrate (HAuCl4 .3H2O) and AmberliteTM IRN 160 was obtained from Sigma-Aldrich, Bengaluru, India. Ascorbic acid and sodium hydroxide was obtained from 5
Merck, Mumbai, India. All the chemicals used are of analytical grade and used as supplied without any further purification. Standard stock solutions of Mn2+, Al3+, Ni2+, Zn2+, Sn2+, Li+, Co2+, Ca2+, Hg2+, Fe3+ and Pb2+ were prepared by dissolving appropriate amount of corresponding chloride and nitrate salts in de-ionized water with resistivity of 18MΩcm. 2.2 Preparation of gold nanoparticles: Ascorbic acid capped gold nanoparticles were synthesized using ascorbic acid as both reducing and stabilizing agent by a method proposed by Tyagi et al.[50] with some modifications. Briefly, 25 mL of 0.25 mM ascorbic acid was taken separately in three glass bottles and pH was adjusted to 5.0, 7.0 and 9.0 mL respectively. After that 450 µL of 10 mM HAuCl4 solution was added drop wise under stirring conditions. An immediate bright red colour appeared indicating the formation of gold nanoparticles. The final pH of synthesis was found to be 3.0, 3.5 and 4.0 respectively. The stability of synthesized AA-AuNPs at various pH (3.0, 3.5 and 4.0) was tested for a period of 30 days by recording absorbance at λmax at regular intervals. 2.3 Characterization of gold nanoparticles: The synthesized nanoparticles were characterized in order to investigate their size, shape and functional group present on the surface. Synergy H1 Hybrid reader (Biotek, Germany) was used to record surface plasma resonance (SPR) spectrum of the synthesized gold colloidal solution. The size and shape measurements were carried out using (Zeiss-Carl-Libra-120) transmission electron microscopy (TEM). The sample for TEM analysis was prepared by drop coating gold nanoparticles solution on a carbon coated copper grid. The size distribution graph was plotted by counting ~ 200 particles. 2.4 Assay for detection and determination of Al3+: The as synthesized nanoparticles were used for the colorimetric detection of Al3+ without any further purification. In order to demonstrate the detection of Al3+, 150 µL of the gold 6
nanoparticles were added to 50 µL of water containing different concentrations of Al3+ ions in a polystyrene 96-well plate and mixed immediately by gently shaking the solution. The UVvisible spectrum was recorded in order to quantify the Al3+ ion concentration using a calibration graph. The calibration graph was obtained by plotting known concentrations of Al3+ ions (0 – 100 ppb) against absorbance ratio A652/A520 nm. The selectivity of the proposed Au NPs preparation for Al3+ was confirmed adding various metal ions Mn2+, Al3+, Ni2+, Zn2+, Sn2+, Li+,Co2+, Ca2+, Hg2+, Fe3+ and Pb2+ followed by analysis using UV-visible spectrophotometer.
2.5 Preparation of oxalate form of anion exchange resin: In order to prepare the oxalate form of anion exchange resin (mean size < 0.300 mm), the microporus resin (AmberliteTM IRN160) was first regenerated by sequentially washing (three times of bed volume) with isopropanol, 1M HCl, 1M NaOH and finally activated with excess amount of sodium oxalate solution. The resin was then washed with de-ionized water till the washings were free from oxalate (checked by Ion Chromatograph). Dionex-ICS-3000 (Sunnyvale, CA, USA) with IonPac-AS-20. Before use 0.5 g of dry resin was transferred into syringe (2 mL capacity, Dispovan, India) to prepare a mini bed. Excess water from the resin was removed under pressure exerted by pressing the piston of the syringe. Resin thus obtained after removal of water is suitable for the column mode of operation, as the volume change due to uptake or release of solvent from the resin was minimal. 2.6 Application of assay for determination of Al3+ in drinking water and ground water: The Al3+ spiked drinking water and ground water samples were used to test the suitability of the assay for Al3+ determination in these matrixes. The water samples were first passed through syringe containing oxalate form of anion exchange resin, before testing for Al3+. In the filtered water samples Al3+ was determined as described the experimental section. 7
2.7 Statistical analysis: Three independent experiments were carried out and the results are represented as mean ± SD. The Student’s t-test was applied to calculate the statistical significance of the experimental data. 3. Results and discussions: 3.1 Synthesis and characterization of gold nanoparticles: Figure 1 represents the results of synthesis and characterization of AA-AuNPs. The red colour appeared within seconds after addition of HAuCl4 to a diluted solution of ascorbic acid. The UV-vis. spectrums of AA-AuNPs synthesized at various pHs (3.0, 3.5 and 4.0) are in shown in Fig. 1a. Appearance of bright red colour and a peak at ~ 520 nm in UV-visible spectrum clearly represents the synthesis of gold nanoparticles. This was further confirmed by TEM analysis; the micrograph shows nearly- monodispersed, spherical nanoparticles (Fig. 1b) of average size of 11.2 ± 4.2 nm (Fig. 1c). Since the nanoparticles prepared with 0.25mM of ascorbic acid at pH-4 provided the most sensitive response (section 3.2), the size analysis of this preparation was carried out by TEM. The AA-AuNPs when synthesized at higher pHs were blue to purple in colour and agglomerated within few hours of synthesis. Hence, the AA-AuNPs synthesized at pHs 3.0, 3.5 and 4.0 were chosen for further investigations. The concentration of reductant and pH of the solution is known to affect the size of the nanoparticles. The stability assessment of the AA-AuNPs synthesized at pH 3.0, 3.5 and 4.0 was carried out by measuring absorbance at 520 nm for a period of 30 days at room temperature since the stability of the colloidal preparation is very important if it has to be used for real time application. The results are presented in Fig. 1d; the graph shows that the synthesized AA-AuNPs remained stable for the tested period without any significant change in absorbance value. 3.2 Assay for detection and determination of Al3+: 8
The as synthesized AA-AuNPs were used directly for the detection of Al3+ without any further purification. In order to demonstrate the assay, 50 µL of water containing Al3+ ions from stock (5 ppm) solution was added to 150 µL of the AA-AuNPs solution in polystyrene microtiter plate wells and mixed by gentle shaking. A change in colour from red to purple was observed almost immediately indicating the aggregation of AA-AuNPs in the presence of Al3+. The UV-visible spectrum of control (without Al3+) and test samples (with Al3+) AAAuNPs were recorded to observe the change in characteristic spectrum of gold nanoparticles. The analyte based aggregation of gold nanoparticles is well established and have been widely used for the selective sensing. The Al3+ based aggregation of AA-AuNPs results mainly due to the interaction of ascorbic acid and Al3+ ions which is possibly more preferred than other metal ions at lower (ppb) concentration. The UV-visible spectrum shows the change of absorbance peak intensity at 520 nm and the appearance of a new peak at 652 nm in the presence of Al3+ (Fig. 2). These kinds of changes in UV-visible spectrum are being associated with the aggregation of gold nanoparticles [22, 51, 52]. A concentration of 0.25 mM of ascorbic acid was used for the synthesis of AuNPs because the AuNPs synthesized at 0.25mM gave the most sensitive response to Al3+. While the AAAuNPs synthesized at 0.1 mM concentration were not stable, and those synthesized at higher concentrations did not improve the sensitivity of the assay. It is important to mention that the detection of aluminium by AA-AuNPs was found to be highly pH dependent. The sensitivity of the method improved with increase in pH from 3.0 – 4.0. As represented in Fig. 3(a ,b) the visual response of AA-AuNPs to aluminium starts at 25, 50 and 75 ppb for the pH – 4.0, 3.5 and 3.0 respectively. Thus a nanoparticles preparation with final pH of 4.0 was selected for further investigations. This fact is further supported by the stability of de-hydro ascorbic acid (DHAA) at pH-4 which has been reported in earlier publications [53, 54].The significant role of DHAA for the formation of chelating complex has been described in detail in the section 9
3.3. The calibration graph was obtained by plotting known concentrations of Al3+ ions (0 – 100 ppb) against absorbance ratio of A652/A520 nm. A linear response was obtained from 20 – 100 ppb of Al3+ with R2 = 0.996. The limit of detection (LOD = 3σ) in deionized water was calculated to be ~ 1.3 ppb. The selectivity of the proposed AA-AuNPs based assay for detection of Al3+ ions was confirmed adding various other transition metal ions Mn2+, Al3+, Ni2+, Zn2+, Sn2+, Li+,Co2+, Ca2+, Hg2+, Fe3+ and Pb2+ followed by analysis using UV-visible spectrophotometer. As it is seen in Fig. 4 that except for Al3+ no other metal ions have shown significant decrease in absorption peak, however in case of iron there was a little increase in the peak intensity when measured by spectrophotometer but visually there is hardly any change in the colour of AuNPs in the presence of Fe2+ as seen in the picture incorporated in the Fig 4. 3.3 Mechanism of selective detection of Al3+ using AA-AuNPs: As shown in the given scheme 1 that the ascorbic acid mediated reduction of gold nanoparticles occurs via de-protonation of ascorbic acid in acidic media (pH 3 - 4) the electrons released in the process reduce Au ions to form gold nanoparticle; these nanoparticles are then capped by de-hydro ascorbic acid (DHAA). The resulting di-ketone form DHAA acts as a strong chelating agent for Al3+ and form oxo-complexes [55], which leads to selective aggregation of gold nanoparticles in the presence of Al3+ [Scheme 2]. Hence, a single reagent i.e. ascorbic acid works as a reductant and a stabilizer for AuNPs and later as a chelator for Al3+. The evidences of in vivo chelating effects of ascorbic acid against aluminium have been reported in the literature, for protecting against Al toxicity. In addition the effect of ascorbic acid on the aluminium absorption in the mice systems has been studied showing that in the presence of ascorbic acid the aluminium absorption was low [56-59]. These studies further suggest strong interaction of ascorbic acid and aluminium. 10
3.4 Application of assay for the determination of Al3+ in drinking water and ground water samples: The developed assay was explored for the determination of Al3+ in the drinking water after spiking with various concentrations (0-100 ppb) of Al3+. Figure 5 shows that the assay worked extremely well and a linear response was observed in the range of 20 ppb - 100 ppb for de-ionized water with R2 = 0.996. In order to further investigate the suitability of the assay in other aqueous matrices the assay was further tested for ground water samples. When this method was applied for the visual determination of Al3+ in different ground water samples, it was observed that in most of the cases, the sensitivity was drastically reduced. Only at concentration levels above 600 ppb level of Al3+, there was response by AA-AuNPs. However, as the maximum permissible limit of Al3+ is ~ 200 ng/mL, it was imperative to know which cations and/or anions normally present in ground water contribute/s to the observed degradation of sensitivity of the visual nano particle reagent. The selectivity of the assay against other transition metal ions has already been established and discussed in previous section. So we, further investigated the interference due to alkali metals, alkaline earth metals and anions present in ground water. In the Deccan region of India, ground water samples were normally found to contain Na+ (164 ± 117 mg/L) K+ (4.6 ± 3.4 mg/L), Ca2+ (110 ± 52 mg/L), Mg2+ (42 ± 21 mg/L) and anions Cl- (186 ± 104 mg/L), F- (1.35± 0.06 mg/L), SO42- (98 ± 68 mg/L), NO3- (99 ± 75 mg/L) [60]. These cations and anions were spiked in to DI water and the degrees of interferences were measured quantitatively using the spectrophotometer. It was observed that among the alkaline earth metals only calcium ion led to significant change in absorption spectrum while other cations did not interfere ( Fig. 6 a). The anions did not affect the absorption characteristic of AA-AuNPs even at the highest concentration tested (supplementary information Fig. 1). Thereafter, efforts were made to 11
investigate the levels at which calcium interference sets in. It was found that the interference begins at Ca2+ concentration ≥15 µg/mL, which could be observed even visually as shown in Fig. 6 b. Other cations and anions did not interfere even when they are present at the maximum level normally present in the ground water.
So in order to remove Ca2+
interference, different anions which are known to form insoluble precipitates with calcium were investigated. Calcium forms insoluble salts with anions like hydrogen phosphate, sulphate and oxalate as the corresponding salts have low solubility products (Ksp) of 2.07 x 10-7, 5.0 x 10-5 and 5.0 x 10-9 respectively. So in order to remove maximum calcium from the ground water, oxalate precipitation was preferred. However, by adding 300 ppm of oxalate while the interference was reduced, the visual method could only produce a colour change at and above aluminium concentration of 200 ppb. Further, the concentration of free oxalate ion present after precipitation of oxalate introduced additional interferences. At a concentration of 150 ppm of oxalate, the interference appeared, and at 600 ppm, the system failed to respond the visual test. These type of experimental observations indicated that a method which offered removal of calcium without introducing any appreciable concentration of oxalate into the test water sample would be preferable. Though, cation exchangers in H+ form could be used to remove Ca2+, such an approach will remove Al3+ due to the higher selectivity of the later towards the cation exchanger. Selective separation and /or preconcentration of cations and anions have been carried out by in situ precipitation on columns packed with ion exchange resins carrying suitable counter ions that would form sparingly soluble salts of the target species in the resin phase. Such selective separation or preconcentration is known as “reactive ion-exchange” (RIEX) [61, 62]. In the present case to separate calcium, oxalate was selected as counter ion of the anion exchange resin as the resulting calcium oxalate precipitates in the resin phase due to its low
12
solubility product. The selective separation of calcium from the water sample passed in the column bed containing anion exchanger (oxalate form) can be represented by: (Rs NR3+)2 (oxalate) + Ca2+X2 (water)→ (Rs NR3+)2 X2 + Ca Oxalate (↓)Rs , Where, Rs = solid resin phase; X, is the anion of the Ca2+ cation, and Ca Oxalate (↓) = the precipitated calcium oxalate. 3.5 Assessment of the performance of the RIEX process towards Ca2+ removal Normally, ground water contains calcium between 70-200 ppm. When 1 mL of the water containing 200 ppm Ca2+ of was passed into the syringe containing the oxalate form of ion exchanger and the obtained water was analysed for calcium by ICP-AES; it was found that the Ca2+ concentration was reduced to 5-7 ppm, table 1(supplementary information), at which the visual method does not experience any interference. As the oxalate form of resin was washed with sufficient water prior to use, so leaching of the oxalate was not expected. However, after precipitation of the calcium oxalate, which is sparingly soluble, the oxalate ion concentration was also checked by ion chromatography. The oxalate concentration in the water was around 4-8 ppm. Thus both the calcium and oxalate concentrations were much below the concentration at which visual interference takes place for the gold nanoparticle. After passing the ground water through syringe containing the oxalate form of ion exchanger it was again tested for its response towards Al3+, the results are being plotted in figure 7. The linear response was observed between 100 - 350 ppb of Al3+ with R2 = 0.996. 3.6 Cross validation of developed assay using ICPMS; accuracy and precision: The developed AA-AuNPs based assay for the detection of Al3+ was further cross validated by comparing the results of the assay to the results obtained by ICPMS. The results are being compared in table 1. As evident from the table that the ground water contained only 10 ppb of Al3+ (ICPMS result) so it was not detected by AA-AuNPs. However when 100 ppb of Al3+ was spiked in the water, the response was clear and the obtained values were in concord with 13
the ICPMS at this level. The precision of Al3+ determination by AA-AuNPs assay in terms of relative standard deviation (RSD) was found to be 3 – 6% (intra-day) whereas as inter-day precision varied between 5 - 8%. As no certified material of Al3+ in ground water was available in our laboratory, the values obtained were compared with ICPMS results. The mean values obtained by both the methods were the subjected to statistical analysis for the comparison. In the two-tailed t-test, pooled standard deviation with degree of freedom (F = 4) was used to find out the calculated t value which was found to be 3.7. Thus, the calculated tvalue (3.7) was found to be less than the critical tabulated t-value of 4.6 (F= 4, p = 0.01). Hence there is no statistically significant difference between the mean obtained by AAAuNPs method and ICPMS results at 99% confidence interval. As discussed in previous section where the application of the developed method in drinking and ground water was tested that the linear range in ground water is 100-350 ppb with LOD of 12.5 ppb thus the method even though is not able to detect very low concentrations of Al3+ as could be done by ICPMS, it can still prove to be highly beneficial for the on-field screening of Al3+ in drinking and ground water systems for the on field screening even though it includes a sample pretreatment step. Since, the step is very simple where the water has to be passed through the REIX resin packed in a syringe, the resultant water can be directly used for the analysis. RIEX is oxalate form of anion exchanger and is prepared in the laboratory and can accompany the detection kit. At the time of test the ground water is passed through RIEX syringe and the treated water (50µL) is mixed with 150 µL of AuNPs, where the colour formation is instantaneous. So the time taken for the measurement is ~ 2 min. Further oxalate resin could be regenerated and reused. On-field and screening of large numbers of water samples using ICPMS is costly and time consuming. 4. Conclusions:
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In conclusion, the report demonstrated a simple, cost-effective colorimetric assay based on AA-AuNPs for the detection of Al3+. The developed method is successfully applied for sensing of residual Al3+ in various aqueous matrices. The sensitivity of method was found to be dependent on pH of the AA-AuNPs preparation, and the nanoparticles synthesized at pH 4.0 were found to be most sensitive. The method was found to be highly selective for Al3+ this was evident due to absence of aggregation of AA-AuNPs by any other alkali metals, alkaline earth metal and transition metal ions and anions tested. The only interference caused by Ca2+ at ≥ 15 ppm concentration could easily be removed by passing the water through REIX resin. The developed method can find enormous application in selective quantification of Al3+ in routine laboratory analysis or rapid on-field assay.
Acknowledgement: The authors would like to thank Dr. Sunil Jai Kumar, Head, NCCCM/BARC, and Dr. D. Karunasagar, Head, EACS, NCCCM/BARC, for his constant support and encouragement.
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Lori Rastogi obtained her M.Sc. from the Department of Biotechnology, University of Pune, Maharashtra, India. Presently she is working as scientific officer in National Center for Compositional Characterisation of Materials/Bhabha Atomic Research Center, Hyderabad, India. Her present research interests are development of facile and ecofriendly methods for the size and shape controlled synthesis of metallic (Ag, Au, Cu and Pd) nanoparticles and their application for the detection of environmental toxicants like heavy metals and organic pollutants. Dr. A. Ballal (Ph. D., Molecular Biology) heads the Biological Damage and Repair Section in the Molecular Biology Division, BARC and is affiliated to HBNI as an Associate Professor. He was a Post Doctoral Fellow, Univ. of South Dakota, USA, working on gene regulation in Staphylococcus. Dr. Ballal was awarded the Young Scientist Award (2006) by the Department of Atomic Energy. He received the Homi Bhabha Medal for securing the 1st rank in the Graduate Programme (1997) of the BARC Training School. His research is focused on deciphering mechanisms responsible for overcoming abiotic stresses and he is an avid electron microscopist. Dr. K. Dash obtained his Ph.D. from Osmania University and works as a senior scientist in National Center for Compositional Characterisation of Materials (NCCCM), Hyderabad under Bhabha Atomic Research Center (BARC), Mumbai, India. His research interest is analysis of high purity materials using spectroscopic and chromatographic techniques (ICP-AES/MS, HPLC, IC and GC-MS) and development of novel sample preparation steps using ion-exchange, vapour phase digestion and classical analytical chemistry. He has 35 publications in various international journals.
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Figure legends:
Figure 1: (a) UV-visible spectrum of AA-AuNPs synthesized at various pH, Inset: Digit photograph of colloidal solution, (b) TEM micrograph, and (c) a bar graph showing size distribution of AA-AuNPs synthesized at pH – 4.0, (d) the time (days) versus absorbance graph of AA-AuNPs synthesized at various pH at room temperature for a period of 30 days.
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Figure 2: UV-visible absorption spectrum of AA-Au NPs solution as a response to Al3+. Inset: Visual colour change as a response of addition of Al3+ to AA-Au NPs solution.
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Figure 3: Response curve of AA-Au NPs to Al3+ as a function of pH. (a) Ratio of A652/ A 520 vs. wavelength has been plotted, (b) Digital photograph of the AA-Au NPs after addition of Al3+ at different pH.
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Figure 4: UV-visible spectrum of AA-Au NPs showing its selectivity against other transition metal ions.
28
Figure 5: A plot between ratios at A652/ A 520 versus various spiked concentrations (0 – 100 ppb) of Al3+ in drinking water.
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Figure 6: (a) Absorbance spectrum of AA-Au NPs after addition of alkaline earth metals, (b) photograph showing the Ca2+ interference at various concentrations (0- 25 ppm).
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Figure 7: (a) UV-visible spectrum of AA-Au NPs as a response to Al3+ concentrations in ground water matrix after pre-treating water with RIEX process. (b) Plot between ratios at A652/ A 520 versus various spiked concentrations (100 – 350 ppb) of Al3+ in ground water.
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Scheme 1: Schematic representation of AA-AuNPs aggregation in the presence of Al3+.
Scheme 2: Mechanism for the aggregation of AA-AuNPs via formation of oxo-complex between Al3+ and dehydroascorbic acid [55].
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Table 1: Comparison of results obtained by ICPMS and AA-AuNPs
Sample ID G.W.1 G.W.2 G.W.1 spiked with 100 ppb Al3+ G.W.1 spiked with 200 ppb Al3+
Concentration of Al3+ (ppb)* ICPMS AA-AuNPs 10.0 ± 1.5 -13.5 ± 0.7 -114.0 ± 1. 8 124.3 ± 3.6 207.5 ± 4.2
204.7 ± 5.9
* (Analysis based on three independent samples, n = 3: Values: Mean S.D.
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