Chloroauric acid trihydrate (HAuCl_4·3H₂O, 99.99 %) was purchased from Alfa Aesar (Ward Hill, MA, USA). Sodium tetrahydroborate (NaBH₄, 99 %) and GSH in the reduced (GSH, 98 %) form were obtained from ACROS ORGANICS (NJ, USA). All other chemicals used in this study were of analytical reagent grade and used without further purification. A JL RO100 Millipore-Q Plus water purifier (Millipore Co., Billerica, MA, USA) supplied deionized water with a resisitivity of 18.25 MΩ·cm.

Au NCs were synthesized using the reported protocol, with a few modifications 17 . To a 50 mL methanol solution (0.5 mM) of HAuCl_4·3H₂O, 1.0 mM GSH (1:2 molar ratio, the total volume of methanol was 50 mL) was added. The mixture was cooled to 0 °C in an ice bath for 30 min. An aqueous solution of NaBH₄ (0.2 M, 12.5 mL), cooled to 0 °C, was injected rapidly into the above mixture under vigorous stirring. The mixture was allowed to react for another hour. The resulting precipitate was collected and washed repeatedly with methanol through centrifugal precipitation. Finally, the Au NCs precipitate was dried and collected as a dark brown powder.

The above clusters (1 mg) were dissolved in 1 mL of deionized water that contained 0.0125 mol of GSH. The mixture reacted at 65 °C ± 5 °C for 24 h at 500 rpm in the reaction container then the supernatant was put in the dark at room temperature for 5 to 8 days. The products were precipitated by added methanol (volume ratio 1:1), then the precipitate was collected and washed repeatedly with methanol through centrifugation. Finally, the products were dried by a vacuum drier and can be dissolved in water easily.

Au NCs dissolved in deionized water were used directly. HRTEM specimens were prepared by drop costing one or two drops of the solutions onto an ultra-thin carbon-coated film supported on a copper grid. Bright field HRTEM images were acquired with an electron microscope operated at 200 kV (JEM-2100HR, JEOL, Japan). Typical magnifications of the images were × 200,000 and × 400,000. Moreover, energy-dispersive X-ray spectroscopy (EDS) was taken three points a sample at the same time using the affix of JEM-2100HR IET250.

UV–vis absorption of the Au NCs was recorded in aqueous solution at ambient temperature using a PerkinElmer Lambda 25 spectrophotometer (PerkinElmer, Waltham, MA, USA). The PL spectra were obtained by a spectrofluorometer (F900, Edinburgh Instruments Ltd., West Livingston, UK). The sample solutions were not deaerated prior to the measurement. The cluster concentrations for the PL measurement were typically <10 μM, where the intensities increase linearly with the cluster concentrations. The PL quantum yield was determined according to 15 .

Mass spectrometric studies were conducted using an electro spray (electrospray ionization mass spectrometry (ESI-MS)) system (LC-6A, Japan). Samples with a concentration lower than 10 ppm, taken in deionized water, were electrosprayed at a low rate of 10 μL/min and an ion spray voltage of 5 kV.

A series concentration of aqueous Hg²⁺ was added to cluster solutions one by one and equilibrated at room temperature for 10 min prior to measurement of the PL with an excitation wavelength at 470 nm. All other ions (Pb²⁺, Cu2+, Ca²⁺, Mg²⁺, K⁺, and Na⁺) tested in interference studies using the same method.

As Hg²⁺ and Pb²⁺ are highly toxic and have adverse effects on human health, all experiments involving heavy metal ions should be performed with protective eyeglasses, chemical safety gloves, and protective clothing to prevent skin exposure. The waste solutions containing heavy metal ions should be collectively reclaimed to avoid polluting the environment.

The well-defined GSH-protected Au NCs were further etched by GSH in different mass ratio of Au NCs and GSH in aqueous solution at various temperatures. The as-prepared and etched Au NCs were detected by UV–vis absorbance and PL spectroscopies. As shown in Figure 1a, the absorption spectrum of Au NCs exhibited a strong and broad absorption band at approximately 500 nm, which indicated that the particle size of Au NCs was smaller than 2.5 nm 20 . As shown in Figure 1b, the absorption spectrum of the etched Au NCs has no absorption band after natural etching at RT. The spectral behavior of the Au NCs is in sharp contrast to that of the etched Au NCs. The reaction of etching leads to a remarkable depression of the UV–vis absorption, which suggests that a transformation of the original clusters into extremely small clusters has occurred 21 . Au NCs show no emission (Figure 1d), and the strong emission peak of the etched Au NCs was observed at approximately 650 nm after natural etching at RT (Figure 1c). The etched Au NC solution was clear under visible light and an intense red PL under UV light (365 nm). The maximum PLQY of the as-prepared etched Au NCs was 30 %, which was higher than PL BSA-Au NCs reported by Ying's group 14 . The PL intensity can be stable for more than half a year at room temperature when the clusters were freeze dried.

The optical properties of the etched Au NCs were monitored by UV–vis and PL spectra at different time scales at room temperature. As shown in Figure 2A, the absorption spectrum of the etched Au NCs exhibited a strong and broad absorption band at approximately 670 nm after natural etching at RT for 1 day, and the absorption spectrum of the etched Au NCs has no absorption band after natural etching at RT for 2 to 10 days. There is no feature at the spectrum of the etched Au NCs with relatively lower absorbance compared to the un-eched Au NCs. The result suggests that the Au NC has transformed into smaller etched Au NCs 17 22 . As shown in Figure 2B, the maximum emission peak of the etched Au NCs was observed at approximately 720 nm after natural etching at RT for 1 day, and the maximum emission peak of the etched Au NCs was blueshifted to approximately 650 nm with an obvious change of PL spectrum profiles after natural etching at RT for 2 to 10 days.

HRTEM images of the as-prepared and etched Au NCs for 5 days, as shown in Figure 3, indicate that the particles are well-dispersed. The diameter of the dispersed as-prepared Au NCs is approximately 2 to 4 nm, with a mean diameter of 3.24 nm, and the diameter of the etched Au NCs is approximately 1.0 to 2.5 nm, with a mean diameter of 1.89 nm.

EDS analysis was employed to determine the chemical identities of the etched Au NCs (Table  1). EDS measurements were conducted to confirm the presence of the GSH at the surface of the etched Au NCs. The element analysis of the etched Au NCs with EDS indicates the presence of Au from the etched Au NCs and also C, N, O, and S from the GSH molecules. The high sulfur percentage confirms an existence of a significant amount of GSH molecule at the surface of the etched Au NCs; the ratio of S/Au was determined to be approximately 1.4.

Figure 4 shows the negative-ion ESI-MS spectra of the etched Au NCs in detail. The mass range available is not enough to see the singly charged cluster. Besides, orthogonal ESI-MS does not give intact cluster species. Hence, we could not see the intact Au core. However, we see the presence of -SG, and small fragments of the Au NCs. The peaks at m/z 1094.31, 1223.48, 1465.94, 1708.35, and 1835.45 are assigned to [Au₈SG₃-H+]⁻, [Au₁₄SG₃-3H+]³⁻, [Au₁₉SG₇-4H+]⁴⁻, [Au₁₂SG₉-3H+]³⁻, and [Au₁₄SG₉-3H+]³⁻, respectively. The result shows that Au₁₄ is the major core. Although the data shows a few impurities, it did not influence the application.

The effect of Hg²⁺ on the PL spectra was investigated. As shown in Figure 5, with the increasing concentration of Hg²⁺, the PL intensity of the etched Au NCs (etching for 2 days) was decreased obviously. Meanwhile, the spectral wavelength maximum was blueshifted 5 to 10 nm, which indicates the change of surface states of the clusters since Hg²⁺ ions bind onto the particle surface 23 . These values implied that the etched Au NCs have high sensitivity and reproducibility for Hg²⁺ assay. It may be used to detect trace toxicity metal ions in water due to the high PLQY.

PL quenching of the etched Au NCs was studied with the addition of the same concentration of different ion solvents (Pb²⁺, Ca²⁺, Cu²⁺, Mg²⁺, K⁺, and Na⁺) (0.2 mM). As shown in Figure 6, Pb²⁺ led to decreases in the I 0 − I I 0 value, whereas the other ions exhibited no significant effects under identical conditions. These features contributed to the excellent selectivity toward Hg²⁺ and Pb²⁺, rendering the etched Au NCs suitable for the heavy metal ions detection in water and environmental monitoring.

A PL quenching was observed in the presence of Hg²⁺, which could be explained in terms of electron transfer between the excited etched Au NCs to the Hg²⁺ 24 . The mechanism can be explained using a photoinduced electron transfer process. First, the etched Au NCs and Hg²⁺ ion form a complex. Second, the electrons within the etched Au NCs are firstly excited to the excited state under photo-irradiation. Third, Hg²⁺ can directly intercept one of the charge carriers and is reduced to Hg⁺, which can disrupt the radiative recombination of the holes and the excited electrons and quench the PL of the etched Au NCs. This assumption is supported by the following facts: In this test, GSH was used in competition with Au NCs for Hg²⁺. The PL of Au NCs was not quenched after the addition of the intermixture (1 mM Hg²⁺, 0.1 mM GSH). In a control experiment, GSH showed no influence on the PL of Au NCs in the absence of Hg²⁺, which confirmed that the complex formation occurred between GSH and Hg²⁺. Based on the finding, we concluded that the complex of Au NCs and Hg²⁺ and a photoinduced electron transfer process were the primary mechanism for PL quenching.

The reason of enhanced fluorescence in the presence of Pb²⁺ was very complicated. The spatial confinement of free electrons in the etched Au NCs results in discrete and size-tunable electronic transitions, leading to molecular-like properties. Therefore, the luminescent mechanism for the etched Au NCs would involve relaxed luminescence across the HOMO-LUMO gap 25 . When the Pb²⁺ was added into the etched Au NCs, HOMO-LUMO gap would be decreased 26 . Therefore, the enhanced luminescence of the etched Au NCs was observed. Due to the complicated reaction process, the precise luminescence enhancement mechanism remains unclear, and the detail reaction mechanism was under the way.