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DOI: 10.1039/D5AN00605H (Paper) Analyst, 2025, Advance Article

Unraveling the stability and specific electrochemical sensing of lead ions using copper nanoclusters on sulfur and nitrogen-doped graphene quantum dots

Saisree S., Dhrishya V., Gaurav Meena, Arya S. Nair and Sandhya K. Y.*
Department of Chemistry, Indian Institute of Space Science and Technology, Valiyamala, Thiruvananthapuram 695547, Kerala, India. E-mail: sandhya@iist.ac.in; Tel: +0471-2568551

Received 2nd June 2025 , Accepted 1st August 2025

First published on 1st August 2025

Abstract

Metal nanoclusters (MNCs) exhibit unique electronic properties, quantum size effects, and high catalytic activity; however, their practical efficacy is often limited by poor stability, particularly in the case of less noble and lower cost metals such as copper compared to noble and high-cost metals such as gold and silver. Stabilization strategies involving bulky ligands compromise structural and synthesis simplicity, while the use of small molecules, for instance, thiolate ligands, offers an attractive alternative by promoting structural simplicity and stability via heteroatom interactions. However, their stability remains limited. Thus, here in this work, we report the synthesis of a highly stable thiolate-protected copper nanocluster anchored on sulfur and nitrogen co-doped graphene quantum dots (CuNC@S,N-GQDs), which demonstrates exceptional structural stability for ≥1 year. The NC displays superior electrochemical sensing performance toward Pb(II) ions, accomplishing specific detection with a limit of detection in the picomolar range. The high specificity is ascribed to its strong affinity to Pb(II) because of the sulphur-functional groups in the CuNC@S,N-GQD. Furthermore, the sensor exhibited excellent sensing performance in complex environmental samples, with ∼100% recovery of Pb(II) in all the spiked real water samples, confirming its suitability for environmental monitoring.

1. Introduction

Lead (Pb) is one of the most toxic heavy metal ions (HMIs) and a global environmental pollutant, which is detrimental to human and animal life.1–4 It causes pollution of water resources and toxicity in living organisms due to its persistence and bioaccumulation. Pb affects the overall metabolism of the body by causing damage to almost every organ like the cardiovascular system, kidneys, liver, bones, hematopoietic system, reproductive system and nervous system.5 According to the World Health Organization, individuals with a blood-Pb concentration greater than 50 ppb are at high risk, and Pb exposure caused 1.5 million deaths worldwide in the year 2021.6 Thus, analysis of Pb ions in lower concentrations is very important relative to environmental protection and health care. There are numerous conventional techniques7–10 but they require expensive experimental setup, have a high cost for single sample analysis, require elaborate sample preparation, have high detection limits and are time-consuming. With rising pollution levels, there is an escalating need for highly sensitive, real-time devices for HMI sensing.11 The electrochemical (EC) method of detection is advantageous12–15 when compared to the aforementioned techniques, due to the latter's tailoring and tunable properties such as higher sensitivity and specificity, super-fast response, and easy sample preparation.16–18 There are numerous electrode materials which have exploited the sensing of Pb(II) ions such as graphene-based materials,19,20 carbon nanofibers,21 multiwalled carbon nanotube (MWCNT),22 metal nanoparticles,23 metal oxides,24 etc. There are only a few reported procedures for Pb(II) ion sensing using graphene quantum dots (GQDs); most of them have higher detection limits and the methods for synthesis of the materials are more complex. Ou et al.25 reported a nanocomposite based on Chitosan-GQDs for the EC sensing of Pb(II) but this method required a pre-concentration step and the limit of detection (LOD) was in the nanomolar range. Zhou et al.26 illustrated a greener approach for synthesising GQDs using graphene oxide L-ascorbic acid (L-AA) for selective sensing of Pb(II), which exhibited nanomolar detection theoretically.

Our group recently reported nitrogen (N)-doped graphene quantum dot stabilised CuNC@N-GQDs,27 which exhibited a remarkable stability of ≥1 year, which is higher compared to the literature reports of CuNCs28 and is attributed to the electron richness of N-GQDs that can donate electrons to CuNCs thus limiting their oxidation. The CuNC@N-GQDs displayed EC sensing of three different neurotransmitters/simulants, attributed to the N-functional groups, which interacted with the N-containing neurotransmitters.27 Inspired by this work, herein, we have synthesized a CuNC using a sulfur (S) co-doped N-GQD (CuNC@S,N-GQD) that may be capable of exhibiting commendable stability, because of the lower electronegativity of S, which makes the donation of electrons by the GQD to the CuNC easier. Furthermore, the S-functional groups have an affinity to Pb(II)29 which can be manipulated fruitfully toward its sensing. Interestingly, the formed CuNC@S,N-GQDs exhibited commendable stability and not only enhanced the sensing capability towards Pb(II) compared to CuNC@N-GQDs, but also imparted specificity towards Pb(II) in the presence of other common toxic metal ions, including Cd(II) and Hg(II), compared to S,N-GQDs. Thus, this work establishes how changes in structure and heteroatom doping can impart, enhance and change sensing properties of a material. The formation of a CuNC on the S,N-GQD alters the material's properties, enabling specific detection of the heavy metal ion Pb(II), which is explored further and discussed in detail in this work based on the interactions and structural features.

2. Experimental section

2.1. Reagents and materials

Aniline, ammonium peroxydisulfate (APS), chloroform, hydrochloric acid (HCl), sodium hydroxide (NaOH), glutathione (GSH), HgCl2, Zn(NO3)2, AgNO3, NiSO4, CdCl2, Co(NO3)2, Pb(NO3)2, FeSO4, SnCl2, CuSO4 and K2Cr2O7 were purchased from Spectrochem Pvt. Ltd. All reagents purchased were of analytical grade and were used without further purification. Distilled water was used in all the procedures unless specified otherwise.

2.2. Characterization

High-resolution transmission electron microscopic (HR-TEM) studies were done on a JEOL JEM 2100 electron microscope with 200 kV accelerating voltage. A CARY 100 Bio UV-visible spectrophotometer was used to obtain the absorption spectra data. A Horiba FluoroMax-4 spectrometer was used to obtain the photoluminescence spectra, with an excitation slit of 1 nm and an emission slit of 2 nm. A Bruker Q-time-of-flight (COMPACT) mass spectrometer was used to deliver the mass spectral data through electron spray ionisation in the negative ion mode with a quadrupole ion energy of 5 eV and a collision energy of 10 eV with a transfer time of 120 ms. A water–methanol mixture of 50[thin space (1/6-em)]:[thin space (1/6-em)]50 ratio was used as the electrospray solvent. X-Ray photoelectron spectroscopy (XPS) analyses were carried out using a PHI 5000 Versa Probe II spectrophotometer (ULVAC-PHI Inc., USA), equipped with a micro-focused (200 mm, 15 kV) monochromatic Al-Ka X-ray source of 1486.6 eV.

2.3. Synthesis of S,N-GQDs and CuNC@S,N-GQDs

The detailed synthesis procedure for S,N-GQDs has been reported elsewhere30 by us. Briefly, from the precursor polyaniline, following a simple acid-catalysed hydrothermal route resulted in crystalline S,N-GQDs with aromatic graphitic basal planes. Furthermore, CuNC@S,N-GQDs were synthesised, and for that, 1 mL of 0.17 M CuSO4 solution was added to 3 mL of GSH (25 mg mL−1) and mixed well for 10 minutes. The reaction mixture became turbid due to the copper thiolate complex formation. Then, 6 mL of the synthesized S,N-GQDs was added to the reaction mixture with constant stirring, and then it was kept at 65 °C with constant stirring for 3 h. The procedure followed was a modified procedure for the synthesis of CuNC@N-GQDs.27 The turbid solution gradually changed to a clear one, indicating the formation of CuNC@S,N-GQDs. Then the whole solution was permitted to cool down to room temperature. The solution attained was centrifuged at 14[thin space (1/6-em)]000 rpm for 20 minutes, and the supernatant solution obtained was stored at 4 °C for further studies.

2.4. Modification of the working electrode

The glassy carbon electrodes (GCEs), with a diameter of 3 mm were mechanically polished with a moistened micro cloth encompassing powder alumina (0.05 mm micro polish powder from CH instruments) and then cleaned cautiously for 2 min in distilled water by ultrasonication before modification with the material. After every analysis, GCEs were polished as demonstrated above and cleaned using distilled water by ultrasonication, followed by acetone. The materials were applied dropwise on the cleaned GCE with attention and then permitted to dry at room temperature for 24 h to attain the modified working electrodes.

2.5. Samples for mass spectrometric analysis

The mass spectrometric analyses were carried out by electrospray ionization time-of-flight mass spectrometry (Bruker Q-time-of-flight (COMPACT) mass spectrometer, USA). The CuNC@S,N-GQD and S,N-GQD sample solutions were diluted 10 times using the solvent system of water/methanol at a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) ratio. The experiments were carried out in the negative ion mode.

2.6. Electrochemical methods

The EC performance of the analytes (Hg(II), Zn(II), Cd(II), Ni(II), Ag(I), Co(II), Mn(II), Fe(II), Cr(VI), Mg(II), and Pb(II)) on the surface of the CuNC@S,N-GQD/GCE was investigated using cyclic voltammetry (CV), and differential pulse voltammetry (DPV) studies at room temperature. The parameters for DPV include a modulation amplitude of 0.025 V and a step potential of 0.005 V, with a modulation time of 0.05 s. A PG 302N, AUT 83909 potentiostat/galvanostat (Metrohm, Autolab, Netherlands) system was used for all the EC measurements in conjunction with the three-electrode system entailing a modified 3 mm diameter GCE, an Ag/AgCl (saturated KCl) reference electrode and a platinum wire counter electrode. 20 μL of CuNC@S,N-GQDs was drop-cast over the GCE, then permitted to dry at room temperature for 24 h; it was then placed in the supporting electrolyte, 0.1 M NaOH, unless specified otherwise.

For the real sample studies, the standard addition method was followed, where identified amounts of Pb(II) were added to actual environmental samples.

3. Results and discussion

3.1. Synthesis and stability of CuNC@S,N-GQDs

CuNC@S,N-GQDs were synthesized following a modified procedure of CuNC@N-GQDs.27 In that work, we observed that N-GQDs, owing to their π-electron-rich structure, not only facilitated the reduction of Cu(II) to Cu(0), but also enhanced the stability of CuNC@N-GQDs by getting incorporated into the system. Therefore, we explored the synthesis of CuNCs using S,N-GQDs and GSH. The choice of S,N-GQDs was motivated by its structural similarity to N-GQDs,16 coupled with the additional benefits offered by the presence of S, which may improve its stability and sensing properties. The synthesis of CuNC@S,N-GQDs comprises (i) a primary step encompassing the formation of a copper thiolate complex from the precursor, and (ii) the reduction step of the copper thiolate, during which the S,N-GQDs are attached to the CuNCs resulting in CuNC@S,N-GQDs, as illustrated in Scheme 1.
image file: d5an00605h-s1.tif
Scheme 1 Illustration of the synthesis and the representative structure of the CuNC@S,N-GQD: the primary step displays the formation of a copper thiolate complex from CuSO4 and the subsequent step, the addition of S,N-GQD to the copper thiolate complex and the formation of CuNC@S,N-GQD.

The structure and stability properties of CuNC@S,N-GQDs were established from their absorption and emission characteristics. The absorption spectrum of CuNC@S,N-GQDs (Fig. 1A) showed peaks at ∼230 nm originating from GSH corresponding to π–π* transitions, at ∼210 nm from the CuNC moiety corresponding to the ligand-to-metal charge transfer, and peaks at ∼260 and ∼620 nm corresponding to π–π* and n–π* transitions of S,N-GQDs.30 The absence of the surface plasmonic resonance peak confirms the absence of nanoparticle formation. The absorption characteristics of CuNC@S,N-GQDs are in good agreement with those of CuNC@N-GQDs (∼230 nm), with an additional shoulder at ∼260 nm, which is from the S,N-GQD moiety. The emission spectrum of CuNC@S,N-GQDs and those of S,N-GQDs and CuNC@N-GQDs are shown in Fig. 1B. The emission characteristics of CuNC@S,N-GQDs exhibit two peaks at ∼480 and ∼524 nm and are attributed to the S,N-GQD and CuNC moieties, respectively. On comparing these peaks with the emission peaks of S,N-GQDs (∼360 and ∼460 nm) and CuNC@N-GQDs (∼330 and ∼430 nm), a red-shift is observed in the λmax value of CuNC@S,N-GQDs, due to the extension of the conjugation in the CuNC@S,N-GQDs, through effective surface functionalisation. The peak characteristics are in good correlation with the emission peak characteristics of CuNC@N-GQDs, further confirming the formation of CuNCs on S,N-GQDs.


image file: d5an00605h-f1.tif
Fig. 1 (A) UV-vis spectra of CuNC@S,N-GQD, S,N-GQD, GSH and CuNC@N-GQD; (inset A) the UV-vis spectra of the S,N-GQD; (B) the emission spectra of the CuNC@S,N-GQD, S,N-GQD and CuNC@N-GQD; (C) and (D) the UV-vis spectra and emission spectra of the CuNC@S,N-GQD, before and the after ∼1 year of storage, respectively. (E) Digital images of the emissions of the CuNC@S,N-GQD before (a and b) and after ∼1 year of storage (c and d) under normal light (a and c) and at UV (365 nm) irradiation (b and d).

It was observed that the temporal stability of CuNC@S,N-GQDs is ≥1 year, and this was demonstrated by the absorption and emission characteristics before and after ∼1 year (Fig. 1C and D). The digital images of the fluorescence emission at the beginning (a and c) and at the end (b and d) of a one-year period are presented in Fig. 1E. The retention of the absorption and emission intensities after one year with respect to the initial ones were ∼95 and ∼92%, indicating a highly stable CuNC@S,N-GQD. The lack of temporal stability is a main limitation for the CuNCs and their applicability.31 The incorporation of a S,N-GQD into the CuNC@S,N-GQD is attributed as the cause of the higher stability. The S,N-GQDs through their electron-rich aromatic basal planes containing conjugated π-electrons and non-bonded electron pairs, play a crucial role in stabilizing the CuNCs and preventing their oxidation, as explained in detail elsewhere.27 This stabilization addresses one of the primary challenges associated with CuNC instability.

3.2. Size, and characteristic properties of CuNC@S,N-GQDs

HR-TEM images of the CuNC@S,N-GQDs are presented in Fig. 2A. The lower-magnification images (Fig. 2A) reveal details about their size distribution and uniformity. The crystalline lattice fringes observed in Fig. 2B display an interlayer spacing of 0.21 nm, corresponding to the (102) diffraction planes of sp2 graphitic carbon,32 which is associated with the spacing of the (102) C–C planes of the S,N-GQD. Additionally, lattice fringes with spacings of 0.18 nm and 0.20 nm (Fig. 2B) are attributed to the (200) and (111) planes of Cu0, respectively, reflecting the interatomic spacings between successive (200) and (111) Cu–Cu planes.27 These observations align with previously reported values for Cu0. The histogram (Fig. 2C) derived from Fig. 2A indicates that the average size distribution of the CuNC@S,N-GQD ranges from 4 to 5 nm.
image file: d5an00605h-f2.tif
Fig. 2 (A) HR-TEM of the synthesized CuNC@S,N-GQD; (B) TEM revealing the d-spacings; (C) the size distribution of the CuNC@S,N-GQD as obtained from the TEM images; (D) and (E) electrospray ionization mass spectrometry time-of-flight mass spectra of the S,N-GQD and CuNC@S,N-GQD, respectively; (F) and (G) expanded negative mode electrospray ionization mass spectrometry time-of flight mass spectra of the CuNC@S,N-GQD at m/z 1604, showing that the experimental spectrum (red trace) is in good agreement with the calculated mass spectrum (black trace), respectively; (H) the possible structure of the CuNC@S,N-GQD.
3.2.1. Proposed structure of the CuNC@S,N-GQD. The mass spectrum of the S,N-GQD (Fig. 2D) shows a base peak at an m/z value of 597 and that of the CuNC@S,N-GQD at 1604 (Fig. 2E). The experimentally observed result (Fig. 2F) matches well with the calculated mass spectrum (Fig. 2G) of the CuNC@S,N-GQD.27 Thus, the molecular ion peak corresponding to m/z 1604 can be denoted as Cu11(S,N-GQD)GSH(CH3O−)3. The results indicate that the CuNC formed contains one S,N-GQD, and one GSH molecule; a possible structure of the CuNC@S,N-GQD is proposed and is given in Fig. 2H.
3.2.2. Characteristics of the CuNC@S,N-GQD. XPS analysis was conducted further to confirm the presence of Cu and other elements such as C, O, N and S. The survey scan spectrum of the CuNC@S,N-GQD (Fig. 3A) revealed the peaks of C 1s (284.9 eV),33 S2p (169.2 eV),34 N 1s (400.7 eV),16 O 1s (530 eV),35 and Cu 2p (939.7 eV).27 The high-resolution (HR) C 1s XPS (Fig. 3B) exposed the presence of C–C/C[double bond, length as m-dash]C (284.7 eV), C[double bond, length as m-dash]O (286.8 eV), and C[double bond, length as m-dash]N (288.3 eV), and that of O 1s (Fig. 3C) indicated the presence of C–O (529.4), C–O–H (532.6) and C[double bond, length as m-dash]O (533.8). The HR N 1s (Fig. 3D) spectra deconvoluted to pyridinic (400.0 eV) and pyrrolic (401.8 eV) N species, and that of the S 2p peak (Fig. 3E) indicated the presence of S[double bond, length as m-dash]O and −SOn−, n = 2, 3 (∼169.0 and 170.2 eV), functionalities and the lack of a peak at ∼163 eV, equivalent to the thiophene bond, demonstrating that the S-doping is through the side functionalities as −SOn−.30 The intense peaks at 932.1 and 939.2 eV (Fig. 3F) are allotted to the 2P3/2 and 2P1/2 of Cu(0).27 These deliver additional indications of the inclusion of the S,N-GQD in the CuNC formation.
image file: d5an00605h-f3.tif
Fig. 3 (A) Survey X-ray photoelectron spectrum of the CuNC@S,N-GQD; (B, C, D, E and F) high-resolution X-ray photoelectron spectra of C 1s, O 1s, N 1s, S 2p and Cu 2p, with the experimental results (black traces) and their related curve-fitted components (other coloured traces) of the CuNC@S,N-GQD.

3.3. Electrochemical sensing studies of the CuNC@S,N-GQD

Preliminary sensing studies (Fig. S1A) were conducted using the following metal ions: Cr(VI), Ag(I), Hg(II), Co(II), Cd(II), Zn(II), Pb(II), and Sn(II), and the results showed that the CuNC@S,N-GQD exhibited a current response only for Pb(II) ions. Comparison of the result with that of the control (Fig. 4A) showed that the CuNC@S,N-GQD/GCE exhibited a current of ∼98 μA for Pb(II) whereas it was ∼1 μA for the CuNC@N-GQD/GCE, which indicates a ∼98-fold higher current response for the former. The specificity of Pb(II) on the CuNC@S,N-GQD/GCE can be ascribed to the interaction between the S functionality and Pb(II)36 and the enhancement of the current response towards Pb(II) on the CuNC@S,N-GQD/GCE is ascribed to its electron-rich and -donating nature due to the graphene structure and lower electronegativity S atoms. Our earlier work30 has shown that the S,N-GQD is capable of simultaneously sensing Cd(II), Pb(II) and Hg(II) (Fig. S2). During the CuNC@S,N-GQD formation, the N-atom functionalities on the S,N-GQD become masked, N being the affinity point of Cd(II),16 resulting in a lack of Cd(II) detection peaks. The non-sensing of Hg(II) by the CuNC@S,N-GQD can be explained based on the greater affinity of Pb(II) than Hg(II) towards S,34 and this is explained in detail in section 3.4. Thus, the synergy of adsorption of Pb(II) on the surface of the CuNC@S,N-GQD through the S,N-GQD and the electron-rich nature contributed towards the enhanced and specific detection of Pb(II).
image file: d5an00605h-f4.tif
Fig. 4 (A) DPV graphs toward 1 mM Pb(II) in 0.1 M NaOH on the bare GCE, CuNC@N-GQD/GCE, and CuNC@S,N-GQD/GCE; (B) DPV responses of Pb(II) in 0.1 M NaOH on the CuNC@S,N-GQD/GCE for the concentration range of 10−12 M to 10−3 M; (C) the corresponding LDR obtained for the range of concentrations 10−12 to 10−3 M; and (F) DPV curves of Pb(II) show a peak starting at 10−12 M confirming the LOD.

3.4. Sensitivity, LDR, LOD, and mechanism of Pb(II) sensing

The variation of current response with the concentration of Pb(II) on the CuNC@S,N-GQD was validated further using DPV analysis, and the result (Fig. 4B) displayed a rise in the current response with an increase in the concentration of Pb(II). The corresponding correlation curve showed two linear dynamic ranges (LDR): from 10−12 to 5 × 10−5 M (R2 = 0.9865, N = 22) and from 2 × 10−5 to 10−3 M (R2 = 0.9735, N = 17) (Fig. 4C). The comparable linear regression equations are [IPA (μA)] = 0.3962x (μM) + 0.6862 (R2 = 0.9865) and IPA (μA) = 0.0554x (μM) + 42.5743 (R2 = 0.9735) and the sensitivity values (calculations in the SI) obtained were 5.607 and 0.784 μA μM−1 cm−2, congruently. The corresponding LOD value (Fig. 4D) attained was 10−12 M (1 pM). In order to validate the results obtained, LOD analysis was conducted using chronoamperometric analysis as well (Fig. S3). The concentrations of Pb(II) were varied from 10−12 to 10−3 and it can be noticed that a current response was observed for 10−12 M (1 pM), thus validating the result obtained using DPV.

The sensing performance of the CuNC@S,N-GQD/GCE was compared with the EC selective sensors of Pb(II) (Table 1). The use of the stripping technique in the EC sensing metal ions is a common strategy to obtain the oxidation peak of the metal ions, for which a pre-reduction step is required to enable the reduction of metal ions or to enhance the current response as performed in the ZnO@graphene nanocomposite,37 the graphene/PANI/polystyrene composite,38 1-dodecanoyl-3-phenylthiourea (DPT),36 Bi2O3/graphite-carbon inks39 and Au nanoparticles on carbon nanofibers (AuNP-CNF),40 which is a time-consuming procedure and imparts complexity to sensing studies and thus removing some of the advantages an EC sensor has to offer. The CuNC@S,N-GQD does not require a pre-reduction process, and its LOD value of 1 pM aligns well with the lowest LOD reported hitherto,35,41 being one order of magnitude lower than that of the S,N-GQD/GCE (Fig. S4). The oxidation peak without the pre-reduction step is evidence of the spontaneous reduction of Pb(II) on the surface of the CuNC@S,N-GQD and is assigned to the presence of the electron-rich S,N-GQD. The spontaneous reduction of Pb(II) to Pb(0) on the CuNC@S,N-GQD is due to the reducing ability of the S,N-GQD, and it can be primarily attributed to the n electrons from the heteroatoms S and N and the π electrons from the aromatic rings of the S,N-GQD.30 This facilitates the detection of Pb(II) without any pre-reduction step in the oxidation potential sweep.

Table 1 Comparison of the LOD and linear dynamic ranges of the CuNC@S,N-GQD/GCE with other literature reports of selective Pb(II) EC sensors
Materials/electrodes Method LOD (nM) Ref.
Graphene/PANI/polystyrene SWASV 16 32
ZnO@graphene nanocomposite SWASV 3 31
Bi2O3/graphite-carbon inks CCSCP 38 34
MWCNT-ion imprinted polymer/Pt DPV 0.09 42
DPT SWASV 3 33
AuNP-CNF SWASV 99 35
MWCNT-βCD DPASV 4 43
AuCuNC@N-GQD DPV 0.001 41
CuNC@S,N-GQD DPV 0.001 This work

The CuNC@S,N-GQD not only exhibited imparted EC sensing properties towards Pb(II), attributed to the presence of S, but it is interesting to note that it also exhibits imparted selectivity. Compared to the sensing of the S,N-GQD, which exhibited simultaneous EC sensing capability towards Cd(II), Pb(II) and Hg(II),30 the CuNC@S,N-GQD did not exhibit sensing towards Cd(II) and Hg(II), but only towards Pb(II), resulting in selective Pb(II) sensing. This is due to the enrolment of N functionalities during the CuNC27 formation which promotes Cd(II) sensing in the N-GQD16 and Hg(II) is not sensed primarily because Pb(II) has a higher affinity towards S,44 which favours Pb(II) interacting more easily with S than with Hg(II),34 resulting in selective Pb(II) sensing. Thus, the specificity of CuNC@S,N-GQD towards Pb(II) can be interpreted by the affinity between Pb(II) and S through Lewis acid–base interactions.34 The soft Lewis base nature of the S in the CuNC@S,N-GQD has a greater affinity towards the heavy metal ions Pb(II) and Hg(II), because of their soft/medium Lewis acid nature. According to the trends in the Periodic Table, it is obvious that the affinity of S and the metal ions is in the order Pb(II) > Hg(II) because of the former's softer Lewis acidic nature, compared with the medium Lewis acidic nature of the latter.34 All these factors contributed towards the selective sensing towards Pb(II) of the CuNC@S,N-GQD. The XPS survey spectra of the CuNC@S,N-GQD-Pb (Fig. S5A) displays the peaks of C, N, S, O, Cu and Pb. The decrease in the peak intensities of the survey scan spectra of the CuNC@S,N-GQD-Pb compared to that of the CuNC@S,N-GQD is indicative of the presence of Pb(II). The HR XPS S 2p spectrum of the CuNC@S,N-GQD-Pb displays the S 2p peak at 170.31 eV (Fig. S5B) whereas that of the CuNC@S,N-GQD is at 169.25 eV indicating a higher oxidation state of S in the former. The result confirms the S–Pb interaction, where a co-ordinate bond is formed between S and Pb(II), and the possible electron transfer from the S,N-GQD to Pb(II) to form Pb(0). The HR spectrum of Pb 4f in the CuNC@S,N-GQD-Pb reveals the characteristic 4f7/2 and 4f5/2 peaks at 138.61 and 143.47 eV (Fig. S5C), respectively, suggesting the presence of Pb in the Pb(0) state that again confirms the reduction of Pb(II) to Pb(0) in the presence of CuNC@S,N-GQD. Thus, the XPS analysis clearly confirms the S–Pb interaction and the reduction of Pb(II) to Pb(0) by the CuNC@S,N-GQD. A written depiction of the sensing of Pb(II) on the modification of the GCE with CuNC@S,N-GQDs with a remarkable enhancement in the current value without any pre-reduction step is illustrated in Scheme 2.


image file: d5an00605h-s2.tif
Scheme 2 A representative diagram displaying the alteration of the GCE with the CuNC@S,N-GQD and the subsequent spontaneous reduction of Pb(II) to Pb(0) on the CuNC@S,N-GQD/GCE and the corresponding heightened EC sensing of Pb(II) by DPV.

3.5. Interference, reusability, stability, and reproducibility studies

To ensure the real-world applicability of the sensor, parameters such as sensitivity and selectivity must be validated.15,29 The CuNC@S,N-GQD/GCE exhibited a sensitivity of 5.607 μA μM−1 cm−2. The selectivity study was conducted using DPV in a solution containing a mixture of 1× concentrations of Sn(II), Fe(III), Cr(VI), Fe(II), Cd(II), Ag(I), Co(II), Zn(II), Li(II), Hg(II), Ni(II), Mg(II), Mn(II),Cu(II), and Pb(II) (Fig. 5A). The results showed that the current response was the same (98 μA) as that attained in the absence of the metal ions, signifying the non-interference of the other metal ions. The discriminatory interaction and sensing of Pb(II) by the CuNC@S,N-GQD is represented in the bar diagram (Fig. 5B). Furthermore, the reusability of the CuNC@S,N-GQD/GCE was validated by comparison of the current response values obtained for 1 mM of Pb(II) on the CuNC@S,N-GQD/GCE (Fig. 5C) earlier and later for several washes, and the outcomes disclosed that it exhibited 90% retention after 40 cycles of washing. The replicability studies were conducted with five different electrodes, and the current responses toward 1 mM of Pb(II) were scrutinized (Fig. 5D). The relative standard deviation (RSD) calculated for the peak current values of these independently modified electrodes was 1.05%, which disclosed the reproducibility of the sensor. Furthermore, the temporal stability studies of the CuNC@S,N-GQD showed that the proportions of the holding capacity of the current response with respect to the original current response were ∼92 and 83% (Fig. 5E) after one and two months of storage.
image file: d5an00605h-f5.tif
Fig. 5 (A and B) Selectivity studies of the CuNC@S,N-GQD/GCE towards Pb(II) in the presence of various analyte species (1 mM each); (C) DPV curves after immediate washing in twenty and forty cycles showing the reusability of the sensor; (D) peak currents of five independently coated electrodes showing the reproducibility of the CuNC@S,N-GQD/GCE; (E) the DPV profiles obtained for 1 mM Pb(II) on the CuNC@S,N-GQD/GCE showing the stability of the sensor for 30 and 60 days.

3.6. Validation of real-time applicability

In order to validate the real-time applicability of the CuNC@S,N-GQD/GCE for the detection of Pb(II), the studies were extended to actual groundwater and wastewater samples. The corresponding DPV plots (Fig. S6), and the equivalent quantities are provided in Table 2; the retrieval values obtained were in the range of ∼98–100%. The commendable recovery values in all the tested samples indicate the successful applicability of the proposed electrode to determine Pb(II) in actual environmental samples.
Table 2 The recovery results of Pb(II) in actual environmental samples
Sample no. Pb(II) spiked (μM)   Pb(II) found (μM) Recovery (%)a
a The retrieval values obtained were within the RSD of 0.5%.
1 0.5 Groundwater 0.495 99.00
Wastewater 0.498 99.60
2 1 Groundwater 0.986 98.67
Wastewater 0.995 99.56
3 2 Groundwater 1.976 98.82
Wastewater 1.996 99.8

4. Conclusion

In this study, we synthesized a highly stable CuNC using the S,N-GQD as the reducing agent and GSH as the stabilizing agent. Along with the reducing capability, the S,N-GQD has been incorporated into the NC, resulting in the CuNC@S,N-GQD, thus providing stability to the CuNC, with an admirable stability of ∼1 year. The synthesised CuNC@S,N-GQDs exhibited specific sensing towards the hazardous heavy metal ion Pb(II). The specificity lies in the well-known selective interaction between the –SOn– functionalities of the CuNC@S,N-GQD and Pb(II), along with the stronger affinity of Pb(II) with S due to the soft acid/base interactions among them compared to other comparable heavy metal ions, for instance, Hg(II). The LOD obtained was 1 pM, and is equal to the lowest reported hitherto. The excellent reproducibility (RSD = 1.05%), reusability (90%), and stability (92%) of the sensor validated the real-time applicability. In all the tested real samples, the sensor exhibited ∼100% recovery, which underscores the applicability of the sensor in actual scenarios.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

DPV, and Chronoamperometric responses, XPS analysis responses of CuNC@S,N-GQD-Pb and Sensitivity Calculation are included in the SI file. See DOI: https://doi.org/10.1039/d5an00605h.

Acknowledgements

The researchers gratefully acknowledge the financial support from the Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram, extended for the research work.

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