A simple and highly sensitive colorimetric assay for the visual detection of lead and chromium using ascorbic acid capped gold nanoparticles

Abstract

Lead (Pb²⁺) and hexavalent chromium (Cr⁶⁺) are highly toxic pollutants with no safe exposure levels, posing significant health risks globally, especially in developing countries. Current detection methods for these metals are often complex and inaccessible, highlighting the urgent need for innovative approaches. In this study, we present a rapid, cost-effective colorimetric assay utilizing ascorbic acid-capped gold nanoparticles (AuNPs) for the selective detection of Pb²⁺ and Cr^3+/6+ ions at levels recommended by regulatory bodies such as the WHO and EPA. The synthesis of our AuNPs was achieved by reducing gold(III) chloride with ascorbic acid, resulting in stable, negatively charged nanoparticles, as characterized by dynamic light scattering, UV-vis spectroscopy and high-resolution transmission electron microscopy. Our method demonstrated high sensitivity, with limits of detection (LOD) of 5.4 ± 0.25 ppb for Pb²⁺, and 6.3 ± 0.23 ppb for Cr⁶⁺, confirming specificity towards these ions in various water samples. The assay's efficacy was validated in real-world applications, including testing drinking water from multiple sources and assessing the performance of filtration systems. This straightforward assay offers a promising tool for monitoring water quality, enhancing public health initiatives and accessibility to critical environmental testing.

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Introduction

Lead and hexavalent chromium are highly toxic pollutants with no safe exposure levels,^1–4 regulated by organizations like the WHO and EPA. Lead poisoning is particularly severe in developing nations, where the World Health Organization (WHO) estimates that about 240 million individuals are overexposed, leading to over one million deaths each year and underscoring a pressing need to develop technologies to detect and alleviate Pb²⁺ toxicity.^5,6 Current detection methods, though accurate, are limited by complexity and accessibility.^7–9 Gold nanoparticles (AuNPs) serve as versatile colorimetric sensors, detecting hazardous chemicals and ensuring food quality and safety.^10,11 Moreover, functionalized AuNPs find extensive medical applications, including immunoassays, therapies, drug delivery, and bioimaging.^12,13 Despite the extensive use of AuNPs in the detection of toxic metals like lead,^14–18 there are no studies that report on the exclusive and selective detection of both lead and hexavalent chromium at EPA or WHO-recommended levels. Here, we propose a simple, rapid colorimetric assay using non-modified gold nanoparticles and ascorbic acid. Our method detects Pb²⁺ and Cr^3+/6+ ions at levels recommended by regulatory bodies, with visual color changes occurring within seconds. Tested on various drinking water samples, our approach offers a cost-effective and efficient means of detecting these hazardous metals.

At the nanoscale, quantum effects influence nanoparticle properties. As gold nanoparticles decrease in size, a surface plasmon, caused by electron oscillation, emerges. This phenomenon, called localized surface plasmon resonance (LSPR), generates electromagnetic waves along the metal dielectric interface. Changes in nanoparticle size, shape, and dielectric constant, along with chemical composition, alter refractive index in the presence of metal ions, leading to LSPR peak frequency shifts and nanoparticle color changes.^19,20 This frequency shift forms the basis for biosensing, imaging, and other biomedical applications of gold nanoparticles, enabling their versatility in detecting various substances, including toxic metals such as lead and chromium.^21,22

Materials and methods

Materials

Gold(III) chloride trihydrate (HAuCl₄·3H₂O, 99.9%) and L-ascorbic acid (99%) were obtained from Sigma-Aldrich. NaCl, ZnSO₄·7H₂O, NiSO₄·6H₂O, CaSO₄·2H₂O, PbCl₂, FeSO₄·7H₂O, HgCl₂, Na₂HAsO₄·7H₂O, CrCl₃·6H₂O, HCl, and NaOH were obtained from Fisher Scientific, MgSO₄·7H₂O, Al₂(SO₄)₃·16H₂O, and CrO₃ from J.T. Baker Chemical, Cd(NO₃)₂·4H₂O from Mallinckrodt Chemicals, and Cu(NO₃)₂·2.5H₂O from EM Science.

Nanoparticle synthesis

Fresh working solutions of 5 mM HAuCl₄·3H₂O and 40 mM ascorbic acid (AA) were prepared in DI water. The pH of the AA solution was adjusted to 8.0 ± 0.1 with NaOH while stirring. The gold nanoparticles (AuNPs) were prepared by rapidly mixing 12 ml of 40 mM AA with 1.5 ml of 5 mM HAuCl₄·3H₂O and adjusting the volume to 30 ml with DI water. This mixture (0.25 mM HAuCl₄ and 16 mM AA at pH 8) was vortexed for 30 seconds resulting in a royal blue color which when placed in a boiling water bath for ∼5 min, while stirring, led to a purple color change followed by a final ruby red color. We note that boiling the AuNPs solution resulted in a quicker color change to ruby color compared to up to 24 hours if the nanoparticles were left to mature at room temperature or at 4 °C.

Preparation of stock solutions of metal ions

The metal ions tested in this study include Na⁺, K⁺, Ca²⁺, Ni²⁺, Cu²⁺, Mn²⁺, Mg²⁺, Zn²⁺, Fe²⁺, Cd²⁺, Pb²⁺, Hg²⁺, Fe³⁺, Al³⁺, Cr³⁺, As⁵⁺, and Cr⁶⁺ and were prepared in Milli-Q DI water using their respective metal salts. Fe³⁺ was prepared through the oxidation of Fe²⁺ ions in DI water over a period of a few hours, until a yellow tint color appears, the strength of it depended on the concentration used. For the colorimetric assay, AuNPs were added to different metal ions solutions prepared in parts per billion (ppb) concentrations ranging between 10 and 1000 ppb. Images were then taken either at 5 minutes following mixing, or up to 4 hours to see if further visual color change has occurred.

Sample preparation

Stock solutions (500, 50, 5, and 0.5 μM) of metal ions were prepared by weighing 5–15 mg of metal salt and dissolving it in 500–1500 μl, followed by appropriate dilutions in DI water, pH 2. For ionic strength testing, 1.0 M working stock solutions of NaCl, KCl, KI, LiCl, CaCl_2, and MgSO₄ were prepared in DI water, pH 2. Metal ions from stock solutions were added to a plastic cuvette to achieve the desired concentration in a total volume of 1 ml. Typically, 50 μl from a metal ion stock solution, 15 μl of 1 M HCl and 15 μl of 1 M NaCl were mixed with 500 μl of the nanoparticle stock solution and adjusted to 1 ml with DI water. The addition of 15 mM HCl not only improved the detection and sensitivity of our assay, but also helped to dissolve insoluble metal oxide particles.

Characterization of ascorbic acid capped gold nanoparticles (AuNPs)

The average particle size from dynamic light scattering (DLS) and surface charge from zeta-potentials (Z) were measured using a zeta-potential PALS analyzer (Brookhaven Institute Co.) at 25.0 °C with n = 6. The surface morphology of the AuNPs before and after exposure to metal ions was evaluated using high-resolution transmission electron microscopy (HR-TEM, JEOL JSM-2010) operated at an accelerating voltage of 200 kV. The samples were deposited on a TEM copper grid (obtained from Ted Pella) and were dried under vacuum before analysis. The average particle size was determined from HR-TEM images using an open-source image-processing program ImageJ (http://imagej.net/). The absorbance spectra of the AuNPs solutions were then characterized by UV-vis spectroscopy on a Varian Cary 50 Bio or Cary 60 spectrophotometers from Agilent Technologies in the range of 350–900 nm and shows the characteristics SPR peaks around 520 nm.¹⁹ The concentration of the gold nanoparticles solution was estimated to be 0.41 nM.²⁰

Limit of detection (LOD)

To determine the limit of detection (LOD) and its uncertainty, UV-vis calibration curves were generated from the mean of 2 to 3 runs, and the corresponding regression analysis for hexavalent chromium and lead were determined using eqn (1) and (2), and established protocols.²³ All samples were prepared in a 50/50 v/v dilution of AuNPs with DI water in the presence of 15 mM NaCl and 15 mM HCl, and immediately scanned on the UV-vis spectrophotometer.In eqn (1) and (2), σ_regression is the standard deviation of the linear regression (or RMSE, root mean square error), m is the slope of the calibration curve and SE_slope is the standard error in the slope.

Colorimetric detection using smartphone

Standard curves for Pb²⁺ and Cr⁶⁺ were prepared by mixing cation concentrations of 0, 5, 10, 15, and 20 ppb with gold nanoparticles in 1 ml cuvettes. The mixtures were incubated in the dark at room temperature for 10 minutes. To evaluate the detection capability of the AuNPs using a smartphone, each sample was photographed under consistent ambient lighting against a white background to minimize external light interference, using a Samsung Galaxy S22 Ultra. Images were captured from the same region of interest (i.e. the center of each sample) and analyzed using ImageJ software (version 1.53), with the mean RGB values plotted against the metal ion concentrations, followed by linear regression analysis.

Results and discussion

Gold nanoparticles: synthesis and characterization

Our AuNPs were synthesized using ascorbic acid, acting as both a reducing and capping agent, determining their size and shape. These nanoparticles are stabilized by repulsive forces, mainly electrostatic repulsion from surface hydroxyl groups, preventing aggregation in water. Aggregation alters the electric field and surface charge, reflected in zeta potential values. Our negatively charged AuNPs showed a shift in zeta potential from −15.0 mV in the absence of metal ions to −10.7 mV and −4.2 mV in the presence of Pb²⁺ and Cr⁶⁺ cations, respectively.

Fig. 1 displays the UV-vis spectrum and HR-TEM images of our AA-capped gold nanoparticles. The absorbance peak at 525 nm corresponds to surface plasmon resonance, giving the nanoparticles a ruby-red color.

Fig. 1 (A) UV-vis spectrum of the SPR peak and appearance of our AuNPs before (a) and after (b) maturation. (B) HR-TEM images and (C) size distribution histogram of the AuNPs before and after induced aggregation by Pb²⁺ (D) and Cr⁶⁺ (E). Total particles count for (C) is 112.

HR-TEM shows mono-dispersed, spherical nanoparticles with an average size of 17 ± 6 nm. High magnification reveals lattice fringes with a characteristic interplanar distance of 0.144 nm, confirming successful AuNP preparation.²⁴ The characteristic interplanar distance of 0.144 nm observed in high magnification lattice fringes of gold nanoparticles corresponds to the (220) planes in the face centered cubic (FCC) crystal structure of gold. This distance matches the theoretical interplanar spacing calculated for these planes based on the known lattice parameter of gold which is approximately 0.408 nm.

AuNPs assay stability, selectivity, and limit of detection (LOD)

Our freshly synthesized AuNPs demonstrated stability at 4 °C for several days without compromising color or functionality. Our results were collected using either freshly prepared, or 2–5 days old, AuNPs with good reproducibility. We chose to work with a variety of relevant analytes such Na⁺, K⁺, Mg²⁺, Mn²⁺, Zn²⁺, Ca²⁺, Ni²⁺, Fe^2+/3+, Cu²⁺, Al³⁺, Cd²⁺, As³⁺, Hg²⁺ and Pb²⁺ and Cr⁶⁺ because of their different ionic sizes and charges and presence in either bottled water or groundwater sources.^25,26 UV-vis spectroscopy revealed cation-induced aggregation, notably by Pb²⁺ and Cr⁶⁺, leading to a decrease in the characteristic absorption band at ∼520 nm and the emergence of a new peak at ∼720 nm (Fig. 2). The ratio of SPR peaks varied with cation type and concentration, indicating the degree of AuNP aggregation. Testing various cations showed that only Pb²⁺ and Cr⁶⁺ caused significant changes in SPR bands and AuNP color, highlighting the assay's specificity for these ions. Even at 1 ppm concentration, the A730/520 absorbance ratio differed markedly for Pb²⁺ and Cr⁶⁺, compared to other cations (Fig. 2).

Fig. 2 (A) Absorbance scans of Pb²⁺ and Cr⁶⁺ at different concentrations showing the decrease in the characteristic absorption band at ∼520 nm and the emergence of new peaks in the 650–750 nm range. (B and C) Metal ions selectivity plots displaying the A_720/525 ratios as a function of the type and concentrations of metal ions. (D and E) Visual color detection and images of our AuNPs in the presence of 1 ppm and 500 ppb of the listed metal cations. The results were obtained using both freshly synthesized AuNPs and AuNPs stored at 4 °C for up to 5 days.

The optical properties of gold nanoparticles are influenced by factors such as size, shape, surface structure, aggregation state, and the surrounding medium. Higher AuNPs concentrations enhance interaction with target analytes, improving nanoprobe strength, but can obscure small color changes due to a stronger background signal. Lower concentrations improve detection limits but reduce the number of nanoprobes. Thus, optimal detection occurs at intermediate AuNPs concentrations, balancing background noise and aggregation rate. To improve the visual detection of AuNP aggregation induced by cations, we focused on those causing color changes at 500 ppb (i.e. Cd²⁺, Al³⁺, Cr³⁺, Cr⁶⁺, and Pb²⁺), repeating the experiment with 15 mM NaCl and 15 mM HCl. These conditions enhanced visual detection of only Cr³⁺, Cr⁶⁺, and Pb²⁺ down to 100 ppb (Fig. 3). Further dilution in DI water (50/50 v/v) further improved detection down to 30 ppb for Cr³⁺, 40 ppb for Cr⁶⁺, and 20 ppb for Pb²⁺ due to a lighter AuNP background color (Fig. 3). The limit of detection (LOD) was calculated using the standard deviation of response and slope method²³ using known analytes concentrations of (0–50 ppb) (Fig. 4, Table 1).

Table 1 Linear regression data and LOD values calculated according to established protocols²³ for the Cr⁶⁺ and Pb²⁺ calibration curves. The data represents the average of 4 independent runs

	Lead (Pb^2+)	Hexavalent chromium (Cr^6+)
SE regression	0.017	0.0266
Slope of calibration curve	0.011	0.0139
LOD (ppb)	5.4	6.3
Slope standard error	±5.3 × 10^−4	±5.27 × 10^−4
LOD standard error	±0.25	±0.23

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The results in Fig. 4 and Table 1 show LOD values for Cr⁶⁺ and Pb²⁺ below the upper permissible limits in drinking water set by WHO or EPA. The assay demonstrates high sensitivity, with LOD values of 5.4 ± 0.25 ppb for Pb²⁺ and 6.3 ± 0.23 ppb for Cr⁶⁺. However, a LOD for Cr³⁺ ions couldn't be determined due to the absence of a bathochromic shift in the LSPR peak frequency at ppb concentrations.

Proposed mechanism of AuNPs formation and metal ion complexation

Ascorbic acid has been frequently used in the synthesis of spherical gold nanoparticles, acting as a two-electron donor, and demonstrating efficient reduction of gold(III) complex ions with a 1 : 1 stoichiometry of ascorbic acid to Au(III):

[AuCl₄]⁻ + AA → [AuCl₂]⁻ + AA_DH + 2Cl⁻

where [AuCl₄]⁻ is the tetrachloroaurate ion of the chloroauric acid (HAuCl₄), while AA and AA_DH represent L-ascorbic acid and dehydro-ascorbic acid, respectively.²⁷ The formation of gold nanoparticles was shown to proceed in three consecutive steps involving two one-electron reduction steps of Au³⁺ ions to Au⁺ ions, followed by further reduction of Au⁺ to Au⁰ atoms, serving as nuclei for the growth and formation of the spherical nanoparticles, upon which AA²⁻ anions adsorb (Scheme 1).

Scheme 1 Representation of the AA-capped AuNPs formation process and illustration of the proposed mechanism of metal ions (M^z+) detection using ascorbic acid-capped AuNPs.

The AA surface coordination of the AuNPs through the two hydroxy groups on the lactone ring, and the strong and stable AA surface coordination of the AuNPs through the two hydroxy groups on the lactone ring, and the strong and stable molecular assembly between Pb²⁺ and Cr^3+/6+ cations and AA-capped AuNPs result in their aggregation.^18,28 The change in color of the dispersed AA-AuNPs from their characteristic SPR band at 525 nm to a wavelength >620 nm indicates the aggregation of AA-AuNPs, as seen in the HR-TEM images (Fig. 1), and the Z-potential shift towards charge neutralization. This highly selective and metal-induced aggregation of the AuNPs is due to surface complexation between the metal cations and the functional groups (i.e. HO– of AA) attached to the AuNPs surface interface (Scheme 1).

An earlier study utilized ascorbic acid-capped gold nanoparticles for the selective colorimetric and visual detection of Al³⁺ in ground water.²⁹ Although similar reagents were used, the synthesis of the AuNPs in the previous study differed significantly from our current assay. These differences include variations in pH, the concentration of chloroauric acid (HAuCl₄), the heating during the initial phase of the AuNPs formation immediately after mixing, and the addition of HCl and NaCl for signal enhancement.

Application to real world samples

To assess the practical utility of our AuNP-based colorimetric detection, we applied our assay to real water samples collected from various sources, including a residence in Potsdam, NY, tap water from our laboratories, and fountains across the SUNY Potsdam College campus. We also evaluated the effectiveness of two water filtration systems (Brita™ and ZeroWater™) in removing Pb²⁺ and Cr^3+/6+ contaminants from spiked water samples. Our results showed distinct color changes in all tested samples, indicating contamination (Fig. 5). Notably, ZeroWater™ filtration effectively eliminated contaminant traces, as evidenced by the lack of discernible color change after filtration. Images of spiked samples before and after ZeroWater™ filtration demonstrated its efficiency in removing Pb²⁺ and Cr⁶⁺ cations at ppb levels (Fig. 5). In contrast, Brita™ filtration showed no significant differences before and after filtration (data not shown), suggesting ineffectiveness of the Brita™ filters in removing the tested metal ions. We note that all our tests have been performed in the presence of 15 mM NaCl and 15 mM HCl. As shown in Fig. 2, the presence of 1 ppm of various metal ions had no effect on the assay signal, whether added individually, or combined. Furthermore, the presence of 100 μM cations like Mg²⁺, Ca²⁺, or Cu²⁺ (2.4, 4, and 6.3 ppm, respectively) has no influence on the aggregation of the AuNps, confirming that the aggregation is solely induced by Pb²⁺ and/or Cr^3+/6+.

Fig. 3 Visual color detection and images of the AuNPs in the presence of indicated metal ions at concentrations of 10–40 ppb (A) using the as-prepared AuNPs in the presence of 15 mM NaCl, and 15 mM HCl. (B and C) 5–100 ppb Cr^3+/6+, and (D) 2.5–25 ppb Pb²⁺ after 50/50 v/v dilution of the AuNPs with DI water. Both fresh and 5 day-old AuNPs were used.

Fig. 4 UV-vis spectra of AuNPs in 15 mM NaCl and 30 mM HCl, with 5–50 ppb Cr⁶⁺ or Pb²⁺ cations, and standard curves at A_620/520 for Cr⁶⁺ and A_720/520 for Pb²⁺ as a function of the concentrations of cations. Mean values ± standard deviation from duplicate or triplicate measurements.

Fig. 5 Colorimetric detection of lead in real samples prepared using 50/50 (v/v) AuNPs/deionized water. (A and B) Samples 1, 2, and 3 were obtained from different water fountains on the SUNY Potsdam campus, before and after filtration using ZeroWater™ filters; samples in (C) are obtained from a house in Potsdam, NY (Filtered Tap Water using ZeroWater™ filters), a lab sink (Tap Water), and a water fountain in Stowell Hall, Chemistry department (Water Fountain). (D) Water samples spiked with 50 ppb Pb²⁺ before and after filtration with ZeroWater™ filters. (E–J) photo of spiked samples with 10, 20, and 50 ppb of Cr³⁺(E and H), of Cr⁶⁺ (F and I), and of Pb²⁺ (G and J) cations before (unfiltered) and after (filtered) filtration, respectively, using the ZeroWater™ filters.

Smartphone use for the quantitative detection of lead and chromium ions in water

Smartphone-based platforms have been used to detect heavy metals, molecules, bacteria, viruses, and others,^30–37 demonstrating their potential for on-site testing with additional simple instrumentations. In our study, we attempted to use such miniaturized systems to capture the color change with a smartphone camera as a result of AuNPs aggregation, without the need for time consuming functionalization processes of the AuNPs surface to improve sensitivity and selectivity.

To prove the applicability of our assay for smartphone-based detection, standard curves for Pb²⁺ (0–20 ppb) and Cr⁶⁺ (0–40 ppb) were prepared, and images were captured following the addition of AuNPs (Fig. 6). Each image was split into red, green, and blue (RGB) channels, and the mean grey value intensity within a region of interest was measured using ImageJ software. Our preliminary analysis for Pb²⁺ showed strong linearity across all three channels (i.e. R² value of 0.9781, 0.9476 and 0.9647 for channels R, G, and B, respectively). The decrease in intensity correlating with increasing Pb²⁺ concentrations is likely due to the diminishing color of the AuNPs. A similar result was observed with Cr⁶⁺ (data not shown) although in both cases further optimization is needed to (1) improve the sensitivity for the two metal cations, (2) to standardize image collection protocols and image analysis, and (3) enhance the reliability and user-friendliness of this detection method for at-home use.

Fig. 6 (A) Image of AuNP samples in the presence of indicated Pb²⁺ concentrations, and (B) plot of color intensity obtained by a smartphone as a function of Pb²⁺ concentrations. Conditions: AuNPs were prepared in the presence of 10 mM NaCl, 10 mM HCl and were diluted 50/50 v/v with DI water.

These preliminary results are promising, demonstrating strong linearity in the 0–20 ppb range for lead detection and serve as a proof of concept that a compact device like a smartphone could enable untrained individuals to perform on-site direct testing of lead and chromium in drinking water at a low-cost. While the concept is simple, further development is needed to refine this process into a fully integrated, portable device for practical use. For instance, our study could be significantly enhanced by incorporating a robust, uniform RGB light source, developing machine learning algorithms for image processing and noise reduction, and exploring nonlinear regression methods for more accurate quantification of lead and chromium ions in water using smartphone-based colorimetry.³¹ Achieving quantitative detection of lead in a portable format has been a challenge due to cost and reliance on expensive equipment. Our research illustrates a lab-based innovation with the potential to have a substantial societal impact and improve public health.

Table 2 presents a summary of various studies on the use of gold nanoparticles (AuNPs) in colorimetry-based optical sensors, highlighting the limits of detection, functionalization strategies, and capping agents employed for the detection and quantification of Pb²⁺ and Cr^3+/6+ cations. Notably, there remains a lack of AuNP-based assays or technologies capable of ultra-sensitive detection visible to the naked eye. Those that achieve such sensitivity often rely on costly equipment and complex experimental procedures to meet the sensitivity standards set by agencies like the WHO and EPA. Our study addresses this gap and offers a simple and straightforward method for detecting both Pb²⁺ and Cr^3+/6+ at parts-per-billion (ppb) levels in drinking water. Nonetheless, significant efforts are needed to develop affordable, eco-friendly optical nanoprobes integrated with easy-to-read platforms for effective heavy metal sensing and quantification.

Table 2 An overview of comparative studies published over the past decade (2014–2024) utilizing AuNPs colorimetric sensors based on surface plasmon resonance (SPR) for the detection of Pb²⁺ and Cr^3+/6+ ions

AuNPs + functionalization and capping agent	Target metal ions and limit of detection (LOD)	Reference
Analyte (Pb ^2+ )
AuNPs	18 μM (3730 ng ml^−1)	38
AuNPs + tannic acid	11.3–60 ng ml^−1	39
AuNPs + N-decanoyltromethamine	72.4 ng ml^−1 (350 nM)	40
AuNPs + oligonucleotide	1035 ng ml^−1	41
AuNPs + valine	6300 ng ml^−1, or 96.5 μM (20 ppm)	42
AuNPs + 11-mercaptoundecanoic acid	2 μM	43
AuNPs + maleic acid	0.5 μg l^−1 (2.41 nM)	44
AuNPs + 3-mercaptonicotinic acid 4-aminobenzo-18-crown-6	50 nM	45
AuNPs + gallic acid	10 nM	46
AuNPs + triazole	16.7 nM	47
AuNPs + tris	0.1 nM	48
AuNPs + 4-methylthio-1-butanol	>100 μM	49
AuNPs (+DNAzyme), (+DNAzyme/biotin), or (+ DNAzyme duplex)	20 pM, 50 pM, or 100 pM	50, 51 or 52
AuNPs + poly(styrene-co-maleic anhydride)	30 nM	53
AuNPs + polymer polyethylenimine (PEI)	700 pM	54
AuNPs (+Pb^2+ aptamers), or (+Pb^2+ aptamers and cationic hexapeptide)	1 nM, and 98.7 pM	55 and 56
AuNPs + pyridine-formaldehyde	1–4 μM	57
AuNPs + T and G-rich aptamers	0.57 μM (120 ng ml^−1)	58
Analyte (Cr ^3+/6+ )
AuNPs + cysteamine	0.236 nM (for Cr^3+)	59
AuNPs + 3-merca-ptopropionic acid	0.34 ppb (for Cr^3+)	60
AuNPs + sodium hyaluronate	0.78 nM (for Cr^3+), 2.90 nM (Cr^6+)	61
AuNPs (+meso-2,3-dimercaptosuccinic acid), or (+Tween 20)	0.01 μM (for Cr^3+/6+), or 0.02 μM (for Cr^3+)	62 and 63
AuNPs + gold nanotetrapods	0.5–3 nM (Cr^6+)	64

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Conclusion

We have successfully demonstrated a simple, effective, and highly sensitive and selective colorimetric assay for the detection of Pb²⁺ and Cr^3+/6+ using ascorbic acid-capped gold nanoparticles and no hazardous reagents. Using deionized water samples spiked with Pb²⁺, Cr⁶⁺ ions, we determined a limit of detection (LOD) of 5.4 ± 0.25 ppb for Pb²⁺ and 6.3 ± 0.23 for Cr⁶⁺, which are values lower than the EPA or the WHO's acceptable limits for drinking water. Our assay is quick, cost effective, and does not require expensive instrumentation, lengthy procedures, or surface modification of the AuNPs, and exhibits visual detection with the naked eye down to ∼20 ppb Pb²⁺ and ∼30–40 ppb for Cr^3+/6+. When applied to real water samples and integrated with a smartphone application, our assay shows great promise for on-site testing. It provides a practical, cost-effective solution for detecting two highly toxic metals, lead and hexavalent chromium, in drinking water, and could significantly contribute to addressing water insecurity in vulnerable communities.

Data availability

The datasets generated and/or analyzed during the current study are not publicly available due to privacy/ethical reasons but are available from the authors on reasonable request.

Author contributions

C. H., M. B., and J. P. designed the study and aided in interpreting the results. C. H., M. B, J. P, and A. O. collected data and wrote the manuscript. F. B.-A. edited the manuscript and supervised the project. All authors have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support for this project comes from Cottrell Postbac Award #CS-PBP-2023 021(F. B.-A.) sponsored by Research Corporation for Science Advancement, and from the Eppley Foundation for Research Award #86561 (F. B.-A.).

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Footnote

† These authors contributed equally to this work.

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