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. 2026 Oct 5;16(46):60048–60058. doi: 10.1039/d6ra06384e

The effect of distant substitution patterns present in novel 3-mercaptopropionic acid-based ligands on the structure and optical properties of gold nanoclusters

Bedanta Borah a, Bhaskar Jyoti Barman a, Avishek Sarkar b, Sukesh Shill b, Abdul Wahab a, Apurba Kr Barman a,✉
PMCID: PMC13636290  PMID: 42835609

Abstract

This work highlights the effect of substitution patterns present in capping ligands that are away from the binding site on the structure and properties of gold nanoclusters (AuNCs). As the capping ligands are known to influence the properties of AuNCs, studies in this direction are required to explore and enhance the vast potential offered by nanoclusters. However, only a handful of ligands have been systematically used to date to study this correlation. Additionally, in the majority of these studies, even when using similar/isomeric ligands, the synthesized AuNCs were found to have different compositions and structures, making it difficult to draw a straightforward conclusion about the role of ligands in the properties of AuNCs. Emphasizing this aspect, we synthesized three ligands based on 3-mercaptopropionic acid. The ligand structure was varied by the incorporation of electron-donating groups at positions distant from the binding site. The three AuNCs synthesized using these ligands were characterized with the help of UV-vis spectroscopy, FTIR spectroscopy, fluorescence spectroscopy, XPS, MALDI-TOF mass spectrometry and TGA. Interestingly, all three AuNCs synthesized had similar dominant stoichiometric distributions, which indicated that any change in their properties must be due to the effect of the substitution patterns of the ligands. This allowed us to precisely compare the role of ligands on the optical properties of AuNCs. The clusters were found to be weakly fluorescent in the blue region. We observed that the fluorescence quantum yield and lifetimes of the nanoclusters increased with the electron-donating ability of the substituents.

This work highlights the effect of substitution patterns present in capping ligands that are away from the binding site on the structure and properties of gold nanoclusters (AuNCs).graphic file with name d6ra06384e-ga.webp

1. Introduction

Research on atomically precise metal nanoclusters, especially those of gold and silver, has grown rapidly since the first crystal structure was reported in 2007.1 This increasing trend could be attributed to their unusual optical and photophysical properties that are distinct from metal complexes, nanoparticles or bulk metals.2–4 Photoluminescence (PL) with high photostability and biocompatibility is one of many such fascinating properties. Therefore, they are excellent candidates for biological applications such as biosensing, in vitro and in vivo imaging, and drug delivery.5,6 The structural as well as optical properties of AuNCs are very much dependent on the properties of the capping ligands used.7–10 The luminescence properties of these AuNCs can be influenced by protecting the ligands through ligand-to-metal charge transfer (LMCT) or ligand-to-metal-metal charge transfer (LMMCT).11,12 Other deciding factors for the photoluminescence properties of metal nanoclusters include the metal–ligand ratio, aggregation-induced emission (AIE), kernel size, and compositional modification.5,13–16

For meaningful large-scale synthesis and industrial applications, one needs to develop a ligand system in which the properties of AuNCs can be predetermined. Systematic and comprehensive studies that can correlate the properties of ligands with the physical and chemical properties of AuNCs are very much required. Few studies have tried to correlate ligand properties with nanocluster properties. Only a handful of ligands have been systematically used to date for this purpose.17–24 The research is in its early stages, and the relationship remains blurred. One of the major problems in correlating the properties of AuNCs is the change in cluster composition and structure, even when synthesized from ligands with minute variations. For example, AuNCs synthesized using isomeric o-, m- and p-methylthiophenols led to the formation of Au38, Au40 and Au36 clusters, respectively.17 The same set of ligands even yielded Au40, Au104 and Au130 clusters with different synthetic methods.18 Similarly, Au25 and Au102 clusters were obtained from isomeric m- and p-mercaptobenzoic acid.19,20 3-Mercaptopropionic acid (3-MPA) and related unbranched aliphatic thiol ligands, such as 6-mercaptohexanoic acid and 8-mercaptooctanoic acid, which differ only in carbon chain lengths, were used to synthesize AuNCs. The compositions of the final clusters were dependent on the synthetic procedures that yielded differently sized Au25, Au15, Au8 and Au7 clusters upon using 3-MPA. Similarly, 6-mercaptohexanoic acid and 8-mercaptooctanoic acid yielded Au25 and Au8 clusters. However, in a few studies, the same clusters were obtained using structurally similar ligands. For example, Au15 clusters were obtained by using 3-MPA and 3-mercapto-2-methylpropanoic acid. Au25 and Au8 clusters were obtained when 3-MPA, 6-mercaptohexanoic acid, and 8-mercaptooctanoic acid were used as ligands. One common finding was that the longer the carbon chain of the ligands, the greater the stability of AuNCs. The longer carbon chain also positively influenced fluorescence properties by increasing the quantum yield.21–24 Thus, in most of these studies, the AuNCs synthesized using isomeric/similar ligands were found to have different compositions and structures. These divergent syntheses are also important for exploring the properties of differently sized AuNCs, but it is difficult to state the exact roles of ligands in the properties of AuNCs in a straightforward manner. These changes in properties are likely to have originated from both the influence of the ligand, as well as structural changes. Therefore, it is important to design ligands that are structurally similar and, in turn, can lead to the formation of identical/similar nanoclusters. In such cases, we will be able to precisely relate the properties of nanoclusters to the properties of ligands. More research is needed in this area, which could greatly increase the possibility of finding robust ligand/metal systems with predictable structure–activity relationships (SAR; the relationship between ligand structure and nanocluster properties). We are working towards this goal to establish structure–activity relationships (SAR) between various ligand types and AuNCs/AgNCs.12,25

Based on 3-MPA, we designed three ligands, namely N-phenyl-3-mercaptopropanamide (L1), N-(4-methylphenyl)-3-mercaptopropanamide (L2) and N-(4-methoxyphenyl)-3-mercaptopropanamide (L3), for this work, keeping in mind our aim to investigate a reliable structure–activity relationship. The ligands are designed to include the following structural factors: (i) constant chain length, (ii) presence of a flexible aliphatic region as well as a rigid aromatic ring, and (iii) distant variations in substitution patterns to influence electronic properties (Fig. 1). L1 has an amide group and an aromatic ring that could interact with Au atoms in addition to the –SH group. The aromatic ring is slightly more electron-rich in L2 due to the presence of an electron-donating –CH3 group. L3 has an –OCH3 group, which has more electron-donating ability through resonance. Surprisingly, all ligands yielded AuNCs with similar composition. However, their fluorescence properties varied according to the electronic properties of the ligands.

Fig. 1. Structures of ligands L1, L2 and L3.

Fig. 1

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The synthesised ligands (Scheme 1a) were characterised by 1H NMR, 13C NMR, HRMS and FTIR spectroscopy (see ESI for spectra). Three nanoclusters, namely AuNC–H, AuNC–CH3 and AuNC–OCH3, were synthesized using L1, L2 and L3, respectively (Scheme 1b). The AuNCs were characterized by UV-vis spectroscopy, FTIR spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS), MALDI-TOF mass spectrometry and thermogravimetric analysis (TGA).

Scheme 1. Methods for the syntheses of (a) ligands and (b) AuNCs.

Scheme 1

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2. Experimental section

2.1. Chemicals

Chloroauric acid (HAuCl4, Loba Chemie, 99.8%), 3-mercaptopropionic acid (Sigma-Aldrich, ≥99%), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (Loba Chemie, 99.8%), aniline, p-toluidine (Loba Chemie, 99%), p-anisidine (Loba Chemie, 98%), triethylamine (Loba Chemie, 99%), and sodium borohydride (Loba Chemie, 99%) were purchased and used without further purification.

Reagent-grade deionised water and other organic solvents (methanol, dichloromethane, petroleum ether, diethyl ether and ethanol) were obtained from Merck and Loba Chemie. Some solvents were distilled before use as required.

2.2. Synthetic procedures

2.2.1. Synthesis of L1

S-Trityl-3-mercaptopropionic acid (1 g, 2.88 mmol) was dissolved in 10 mL dry dichloromethane (DCM). 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC. HCl, 1.10 g, 5.76 mmol) in 10 mL dry DCM was added to the reaction mixture, followed by the addition of triethylamine (0.800 mL, 5.76 mmol) at 0 °C. After 30 min, aniline (0.524 mL, 5.76 mmol) was added and stirring was continued for 24 hours. The reaction mixture was then filtered, and the filtrate was concentrated under reduced pressure. The residue was dissolved in DCM, and the organic layer was washed with 1 N HCl, 5% NaHCO3, and brine, then dried over anhydrous sodium sulphate. The crude compound was purified through silica gel column chromatography using a DCM-methanol (97 : 2) solvent system. The yield for this product (N-phenyl-s-trityl-3-mercaptopropanamide) was 0.825 g (67.36%) Rf: 0.7 (5% methanol in dichloromethane). 1H NMR (600 MHz, CDCl3, δ ppm): 7.47 (t, J = 8.7 Hz, 8H), 7.35–7.29 (m, 8H), 7.25 (t, J = 7.3 Hz, 2H), 7.16–7.08 (m, 2H), 2.63 (t, J = 7.3 Hz, 2H), 2.17 (t, J = 7.3 Hz, 2H). 13C NMR (150 MHz, CDCl3.δ ppm): 169.24, 144.61, 137.66, 129.61, 128.97, 128.02, 126.79, 124.32, 119.78, 67.00, 36.84, 27.67. HRMS (ESI) m/z: [M + Na]+ calculated for [C28H25NOSNa]+ 446.1555; found 446.1550.

N-Phenyl-s-trityl-3-mercaptopropanamide (0.500 g, 1.18 mmol) was dissolved in dry DCM (2 mL). Triethyl silane (0.188 mL, 1.18 mmol) was added to the solution and stirred for 5 minutes. Trifluoroacetic acid (2 mL) was then added, and the reaction mixture was stirred for 1 hour. The solution was then concentrated under reduced pressure, and diethyl ether was added. The precipitate formed was dried and washed several times with petroleum ether to remove triphenylmethane and any starting compound to obtain pure L1. The white solid compound was dried in a high-vacuum pump. Yield = 0.152 g (71%) Rf: 0.3 (10% methanol in dichloromethane). 1H NMR (600 MHz, CDCl3, δ ppm) 7.54 (d, J = 8.0 Hz, 2H), 7.34 (t, J = 7.8 Hz, 2H), 7.14 (t, J = 7.4 Hz, 1H), 2.97–2.84 (m, 2H), 2.70 (t, J = 6.6 Hz, 2H), 1.72 (t, J = 8.4 Hz, 1H). 13C NMR (150 MHz, CDCl3.δ ppm): 169.12, 137.56, 129.06, 124.57, 120.03, 41.36, 20.33. HRMS (ESI) m/z: [M + H]+ calculated for [C9H11NOSH]+ 182.0640; found 182.0639.

2.2.2. Synthesis of AuNC–H, AuNC–CH3 and AuNC–OCH3

For the synthesis of AuNC–H, 0.040 g (0.11 mmol) of HAuCl4 was dissolved in 10 mL of methanol. Two equivalents of L1 (0.23 mmol, 0.042 g) were added to the HAuCl4 solution. A turbid pale-yellow complex was formed at this stage. The complex was stirred for 30 minutes, and then two equivalents of sodium borohydride (0.23 mmol, 0.008 g) in 5 mL of distilled water were added dropwise. The solution turned purple immediately and, upon continuous stirring for 24 hours, turned into a darker shade. The synthesized AuNC–H was centrifuged (6000 rpm), and the clear supernatant was discarded. The dark purple nanocluster was collected at the bottom of the centrifuge tube. The crude product was purified by repeated washing with methanol and ethanol. The crude product was first washed with methanol. For this purpose, 2 mL methanol was added to the centrifuge tube, mixed thoroughly for 2–3 min and then centrifuged. The TLC of the supernatant methanol layer was checked for the presence of unreacted ligands. This process was repeated until no unreacted ligand was observed in the TLC. The product was then further purified by washing with ethanol 3 times in a similar manner. The solid compound was then dried under vacuum and stored at 4 °C. The isolated yield was 15.60 mg. The same method was repeated for ligands L2 and L3 for the syntheses of AuNC–CH3 (yield 16.20 mg) and AuNC–OCH3 (yield 15.20 mg), respectively.

The method is reproducible. We synthesized at least five batches at the 50 mg level. The reproducibility between synthesis batches was monitored by UV-vis spectroscopy. We recorded MALDI mass spectra for 3 batches and observed a very similar distribution of peaks for all AuNCs.

2.3. Instrumentation and characterization

UV-vis absorption spectra were recorded using a Shimadzu UV-2600 spectrophotometer. FTIR spectra were recorded using an ALPHA BRUKER FT-IR spectrophotometer in the frequency range of 4000–400 cm−1. NMR analysis was performed using a 600 MHz Bruker Ascend NMR spectrometer. HRMS spectra were obtained using an Agilent-Q TOF 6546 LC instrument using electrospray ionization (ESI) technique in positive mode and methanol as the solvent. Emission spectra were recorded using a Horiba FluoroMax-4 instrument. The photoluminescence decay lifetime measurements were carried out on a time-correlated single-photon counting (Edinburgh Instruments-LifeSpec II) spectrometer. XPS spectra were obtained using a PHI 5000 Versa Probe III (M/s Physical Electronics, USA). The binding energy scale was calibrated using the C 1s core-level signal of aliphatic/aromatic carbon, assigned to 285.00 eV, for all samples. XPS data were analyzed by applying a Shirley-type background correction, followed by peak deconvolution using pseudo-Voigt functions through the OriginPro software. The fitting procedure produced an R2 value of approximately 0.99, indicating good agreement between the experimental and fitted spectra. The FWHM values were constrained for chemically similar species and kept within physically meaningful limits to obtain consistent and reliable peak assignments. The binding energies and peak areas were optimized within reasonable ranges during the fitting process. The Au 4f spectrum shows two characteristic spin–orbit components, Au 4f7/2 and Au 4f5/2, with a separation of 3.6 eV. The mass spectra were obtained using a MALDI-TOF (Bruker, Model: Autoflex speed) mass spectrometer in positive mode, with sinapic acid as the matrix and acetonitrile as the solvent. Thermogravimetric analyses (TGA) of AuNC–H and AuNC–CH3 were performed on a Mettler Toledo instrument with a temperature range of 25–800 °C and a heating rate of 20 °C min−1, with a constant flow of nitrogen gas. For AuNC–OCH3, TGA was performed using a PerkinElmer TGA 4000 thermogravimetric analyzer under similar experimental conditions. PXRD measurements of all nanocluster samples were performed using a Rigaku Ultima IV X-ray powder diffractometer equipped with a Cu Kα radiation source, Ni filter, and D/teX Ultra-high-speed position-sensitive detector. The measurements were carried out at room temperature over a 2θ range of 5–80°, with a scan rate of 10° min−1 and a step size of 0.02°. The Ni filter was used to suppress Cu Kβ radiation.

Additional experimental details are provided in the (SI).

3. Results and discussion

The UV-vis spectra of AuNC–H, AuNC–CH3 and AuNC–OCH3 are consistent with nanocluster formation. All clusters absorbed continuously in the visible and UV regions without any prominent peak that could be assigned to surface plasmon resonance (SPR). Large Au nanoparticles (AuNPs, 5–20 nm) exhibited a single SPR band at around 520 nm. The band progressively blue-shifted for smaller nanoparticles and disappeared in the case of AuNCs (<2 nm). AuNCs possess discrete energy levels and therefore show discrete UV-vis peaks. These peaks are distinctly visible at a low temperature, but at a higher temperature (even at rt) become broad and appear to be a continuous absorption with low-intensity hump peaks.26–28 The UV-vis spectra of AuNC–H, AuNC–CH3 and AuNC–OCH3 exhibit exactly the same pattern, with continuous absorption and weak intensity peaks around 360 nm and 542 nm. All three spectra are almost similar, barring the ligand-specific peaks near 250 nm (Fig. 2). These observations support the formation of nanoclusters and indicate that the three clusters may have similar structures.

Fig. 2. UV-vis spectra of (a) L1 (black) and AuNC–H (red), (b) L2 (black) and AuNC–CH3 (orange) and (c) L3 (black) and AuNC–OCH3 (green) in acetonitrile.

Fig. 2

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The freshly reduced L1, L2 and L3 have peaks at 2559 cm−1, 2577 cm−1 and 2563 cm−1, respectively, in their FTIR spectra, corresponding to the –SH stretching vibration. All clusters lack these peaks, which indicates that the binding of ligands to the Au atoms is most likely through Au–S bonds (Fig. S10–S12, SI).29,30

X-ray photoelectron spectra (XPS) were recorded to determine the chemical compositions and ionic states of the constituent elements present in AuNC–H, AuNC–CH3 and AuNC–OCH3. Survey scans of all clusters showed peaks corresponding to C, N, S, O and Au (Fig. 3a, 4a and 5a).

Fig. 3. (a) Full-scan XPS survey spectrum of AuNC–H. XPS spectra of (b) Au 4f, (c) S 2p, (d) N 1s and (e) O 1s.

Fig. 3

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Fig. 4. (a) Full-scan XPS survey spectrum of AuNC–CH3. XPS spectra of (b) Au 4f, (c) S 2p, (d) N 1s and (e) O 1s.

Fig. 4

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Fig. 5. (a) Full-scan XPS survey spectrum of AuNC–OCH3. XPS spectra of (b) Au 4f, (c) S 2p, (d) N 1s and (e) O 1s.

Fig. 5

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The peak position and width of Au 4f7/2 have been correlated to the size and electronic structure of AuNCs. The standard metallic gold foil, with Au at 0 oxidation state, shows this peak at 84.0 eV or lower. This value shifts towards higher binding energies from metallic gold to AuNPs to AuNCs. For the AuNCs comprising 8 to 44 atoms, these binding energy (BE) values are in the range of 85.1 eV to 84.3 eV. Another important feature is the width of these peaks. The peaks are sharper for metallic Au and AuNPs, with distinctly separated 4f7/2 and 4f5/2 peaks, but are broader for AuNCs, and where peaks may overlap with one another. In the case of Au2O3, where the Au atoms are in the highest oxidized state, the 4f7/2 peaks are observed in the higher binding energy range of 85.8–86.2 eV.31–33 The Au 4f7/2 peaks for AuNC–H, AuNC–CH3 and AuNC–OCH3 were observed at 84.7 eV, 84.7 eV and 85.1 eV, respectively, while the Au 4f5/2 peaks were positioned at 88.3 eV, 88.3 eV and 88.6 eV, respectively (Fig. 3b, 4b and 5b). The peaks are relatively broad compared to the previous reports on Au metal and AuNPs. Thus, the position and width of the Au 4f7/2 peaks observed for AuNC–H, AuNC–CH3 and AuNC–OCH3 are indicative of nanocluster formation, and perhaps the cluster size may fall in the range of Au8–Au25 as indicated by earlier reports.31–33 Also, the formation of any Au oxide particles can be ruled out.

The BEs of S 2p XPS peaks for all three clusters were found at 163.3 eV (Fig. 3c, 4c and 5c). This position confirms binding of the ligands to the metal through an Au–S bond, as unbound –SH, –S–S– or other oxidized forms of S show peaks at greater than 164 eV.34–36 The N 1s XPS peaks for AuNC–H, AuNC–CH3 and AuNC–OCH3 are positioned at 399.8, 399.6 and 399.3 eV, respectively, indicating the presence of unsaturated N atoms that are not bound to the metal atoms (Fig. 3d, 4d and 5d).15,37 In AuNC–H and AuNC–CH3, the O 1s signals were observed at 531.2 and 531.0 eV, respectively (Fig. 3e and 4e). These peaks could be attributed to the C Created by potrace 1.16, written by Peter Selinger 2001-2019 O functional group. For AuNC–OCH3, the broad O 1s peak could be deconvoluted into two peaks: the one at 531.1 eV corresponding to C Created by potrace 1.16, written by Peter Selinger 2001-2019 O and the other one at 532.9 eV for C–O–C (Fig. 5e).38–40 Thus, it can be concluded from XPS analysis that the most probable mode of binding of the ligands with Au is through S atoms present in the ligands.

MALDI-TOF mass spectra were recorded to analyze the mass and composition of AuNCs (Fig. 6a–c). The spectra of all clusters follow an exactly similar pattern. The first set of peaks is found at 1334.50 Da, 1376.06 Da and 1424.09 Da for AuNC–H, AuNC–CH3 and AuNC–OCH3, respectively (Tables T2–T4, SI). The peaks for AuNC–CH3 and AuNC–OCH3 differ by 42 and 90 mass units from that of the AuNC–H cluster. L2 and L3 possess higher masses of 14 Da and 30 Da, respectively, compared to L1. Thus, the observed differences of 42 and 90 mass units in the cluster can be correlated with the presence of three or multiples of three ligands for these peak positions (Table T5, SI). These peaks are best fitted with the mass of Au12L9 clusters. Similarly, another set of peaks observed at 1552.96 Da, 1608.76 Da and 1672.60 Da could be assigned to the formation of AuNCs having either four or multiples of four ligands (mass differences of 56 and 120). The third and last set of prominent peaks found at 1929.54 Da, 1999.47 Da and 2079.34 Da are assigned to clusters with AuNCs comprising multiples of five ligands (mass differences of 70 and 150). Thus, we could assign the formation of Au12L9, Au12L12, and Au15L15 clusters, respectively, for AuNC–H, AuNC–CH3 and AuNC–OCH3. Peaks with multiple charges are common in the mass spectra of AuNCs, and we assigned the observed peaks as 3+ ions in this context.41 Isotopic distributions are also consistent with these assignments (Fig. S13–S15, SI).

Fig. 6. MALDI-TOF mass spectra of (a) AuNC–H, (b) AuNC–CH3, and (c) AuNC–OCH3. (d, e, and f) Zoomed-in views of (a, b and c), respectively.

Fig. 6

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Another interesting feature in the mass spectra is the observation of triplet types of peaks for Au12L12 and Au15L15 (Fig. 6d–f). For example, this triplet appeared at 1537.30 Da, 1552.96 Da and 1560.94 Da, corresponding to the Au12L12 peaks for AuNC–H. The pattern could be correlated with presence or absence of additional solvent molecules. We could interpret these as corresponding to Au12L12 + H2O, Au12L12 + CH3CH2OH + H2O (ethanol was repeatedly used for washing during purification) and Au12L12 + 2CH3CH2OH (Tables T2–T4, SI). Similar observations were made for AuNC–CH3 and AuNC–OCH3 clusters. This implies that a few solvent molecules may be present as adducts in the clusters. Interestingly, this pattern was not observed for the Au12L9 peak. This may imply a structural difference between Au12L9 and Au12L12 clusters. Surprisingly, these patterns are also the same for all three clusters. Thus, we conclude that all three AuNCs synthesized have similar composition and structure.

We also carried out a time-dependent study to see the evolution of nanoclusters. Mass spectra were recorded for 6 sets of reactions at 1 h, 4 h, 8 h, 14 h, 20 h and 24 h, respectively (Fig. S16–S18, SI). At 1 h, the peaks for Au12L12 and Au15L15 were observed for all the nanoclusters, while the peak for Au12L9 was absent for AuNC–H and AuNC–OCH3. However, this peak was consistently observed after 4 h in all three clusters. The overall distributions of peaks are almost constant when we compare the time-dependent spectra. We also recorded the time-dependent UV-vis spectra (Fig. S19, SI). The UV-vis spectrum changed from the start up to 14 h, after which it remained constant. Importantly, the UV-vis spectra also varied with time in the same manner for the three clusters. Due to the consistencies observed in these studies, we propose that the nanocluster product we obtained may be a mixture of Au12 and Au15 clusters. The consistent nature of the time-dependent peaks for AuNC–H, AuNC–CH3 and AuNC–OCH3 is indicative of a similar evolution pathway for AuNCs. Another important observation is the higher intensity of the Au12L12 peak throughout all eighteen spectra recorded. Thus, we propose that the Au12L12 cluster may be the most stable out of the Au12L9, Au12L12 and Au15L15 clusters.

To verify the proposed composition deduced from mass spectra, TGA was carried out to determine the amount of organic matter and Au content in the nanoclusters (Fig. 7). The TGA curves of AuNC–H, AuNC–CH3 and AuNC–OCH3 showed 44.7%, 46.8% and 48.2% mass loss, respectively, which could be due to the loss of stabilizing ligands. 55.3%, 53.2% and 51.8% residues were seen in these TGA curves. We assume these residues to be pure Au based on earlier TGA reports for AuNCs.42–44 Since the AuNCs have mixed compositions, calculation of the exact mass contribution from Au and ligands was not possible. Instead, we calculated the percentage range by taking 100% Au12L9 as one extreme and 100% Au12L12 + Au15L15 as the other. The calculations are shown in Table 1, where it can be seen that the experimental mass percentages of Au and ligands are well within the calculated range for all three clusters. Let us consider the AuNC–H cluster for understanding the theoretical calculations. It is a mixture of Au12L9, Au12L12 and Au15L15. In the case of Au12L9, the mass of 12 Au atoms is 2363.64 Da, while the mass of nine L1 ligands is 1622.25 Da. The calculated mass% of Au is 59.3% and that of the ligand is 41.7. For both Au12L12 and Au15L15, the mass% of Au will be the same, as the Au/L ratio is also the same. In these cases, the calculated mass% of Au is 52.2%, and the percentage of ligand is 47.8%. Now, if the AuNC–H is a mixed cluster, the mass percentage of Au should be in the range of 52.2–59.3%, and the percentage of ligand should be in the range of 41.7–47.8%. The experimentally observed percentages are 55.3% and 44.7% for Au and ligand, respectively. The experimental observations are well within the calculated range (Table 1).

Fig. 7. TGA curves of (a) AuNC–H, (b) AuNC–CH3 and (c) AuNC–OCH3.

Fig. 7

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Table 1. Comparison of the calculated and experimental mass percentages of Au and ligands in AuNCs.

AuNC Calculated mass% of Au in Au12L9 Calculated mass% of Au in Au12L12 and Au15L15 Calculated mass% range for Au Experimental mass% range for Au Calculated mass% of ligand in Au12L9 Calculated mass% of ligand in Au12L12 and Au15L15 Calculated mass% range for ligand Experimental mass% range for ligand
AuNC–H 59.3 52.2 52.2–59.3 55.3 41.7 47.8 41.7–47.8 44.7
AuNC–CH3 57.5 50.3 50.3–57.5 53.2 42.5 49.7 42.5–49.7 46.8
AuNC–OCH3 55.5 48.3 48.3–55.5 51.8 44.5 51.7 44.5–51.7 48.2
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We also recorded the PXRD patterns to compare the bulk nature of AuNC–H, AuNC–CH3 and AuNC–OCH3. The PXRD spectra for the three clusters are also very similar (Fig. S20). All samples showed broad and identically positioned 2θ peaks at 38.2°, 44.3°, 64.6° and 77.6°. These peaks could be assigned to the (111), (200), (220), and (311) crystallographic planes of the face-centered cubic lattice of Au atoms and were observed in previously reported AuNCs. These peaks are generally very sharp for metallic Au and AuNPs, but are broad when observed for AuNCs.45–47

The emission spectra of AuNC–H, AuNC–CH3 and AuNC–OCH3 were recorded in DMSO using an excitation wavelength of 370 nm. As shown in Fig. 8a, all three AuNCs show broad emission bands extending from 380 nm to 500 nm, with multiple emission maxima centred around 400–450 nm. The co-existence of differently sized emissive nanoclusters and multiple emissive surface states arising from metal–ligand interactions is likely the reason for the observation of multiple emission peaks. The observed increasing intensity trend from AuNC–H to AuNC–OCH3 further suggests that the electronic nature of surface ligands significantly influences radiative relaxation processes. The PL quantum yields (QY), determined by the relative slope method using quinine sulphate as a standard, were obtained as 9.07%, 9.64% and 11.56% for AuNC–H, AuNC–CH3 and AuNC–OCH3, respectively (Fig. S25, SI). Further, time-resolved photoluminescence (TRPL) measurements yielded average excited-state lifetimes of 1.22 ns, 1.24 ns and 1.69 ns, respectively, for AuNC–H, AuNC–CH3 and AuNC–OCH3 (Fig. S21–S24, SI). The longer excited-state lifetime of AuNC–OCH3 is consistent with its higher QY, suggesting that the methoxy substituent suppresses non-radiative pathways and promotes efficient radiative decays. This view is supported by the calculated radiative and non-radiative decay rate constants (Table 2). The calculated radiative rate constants are 7.43 × 107 s−1 for AuNC–H, 7.77 × 107 s−1 for AuNC–CH3 and 6.84 × 107 s−1 for AuNC–OCH3, and the corresponding non-radiative rate constants are 7.45 × 108 s−1 for AuNC–H, 7.29 × 108 s−1 for AuNC–CH3 and 5.23 × 108 s−1 for AuNC–OCH3. AuNC–OCH3 shows a significantly lower non-radiative rate constant compared to AuNC–H and AuNC–CH3, while maintaining a comparable radiative rate.

Fig. 8. (a) Fluorescence emission spectra of L1, L2, L3, AuNC–H, AuNC–CH3 and AuNC–OCH3 dissolved in DMSO. (b) Photographs of AuNC–H (i and ii), AuNC–CH3 (iii and iv) and AuNC–OCH3 (v and vi) in normal and UV light, respectively. The measurement uncertainty is ±5%.

Fig. 8

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Table 2. Photophysical data of AuNCs in DMSO at room temperature (300 K)a.

AuNC QY (%) τ (ns) k r (s−1) k nr (s−1)
AuNC–H 9.07 1.22 7.43 × 107 7.45 × 108
AuNC–CH3 9.64 1.24 7.77 × 107 7.29 × 108
AuNC–OCH3 11.56 1.69 6.84 × 107 5.23 × 108
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a

Associated errors of ±5%.

We observed three emerging trends in luminescence properties: (i) QY increases progressively from –H to –CH3 and –OCH3 substituted ligands, indicating that electron-donating substituents enhance the emission efficiency of AuNCs, (ii) excited-state lifetimes show a similar trend, further supporting the reduction of non-radiative decay channels in the presence of stronger electron-donating ligands and (iii) there is a red shift of the first peak from 403 nm to 414 nm in the case of AuNC–OCH3, unlike others. The red shift implies that a stronger electron-donating ligand modifies the electronic structure of the metal–ligand interface, stabilizing the emissive state.

Although the observed differences in the luminescence properties are modest, they are significant because all AuNCs have similar core composition and structure. Therefore, the variations in PL properties most likely originate from differences in electronic properties of the capping ligands. There may also be a contribution from cluster distributions, as the ratio of A12/Au15 clusters may not be identical across the three AuNCs. The results demonstrate that substituents are capable of effectively modulating the optical properties of AuNCs, even when they are far from the Au–S binding sites. Thus, these findings highlight the importance of ligand engineering as a tool for fine-tuning the photophysical properties of NCs without altering the core structure.

4. Conclusions

In conclusion, we synthesized three new ligands based on 3-MPA with distant variation of substituents with respect to the binding site. Three new nanoclusters were synthesized using these ligands. Earlier studies showed that variations in ligand substituent patterns/sizes closer to the binding site led to clusters of different sizes. We emphasized the point of distant substitution to obtain similar clusters but with different properties. We found that distant variation of substituents yielded AuNCs with similar compositions of Au12 and Au15 clusters. Interestingly, distant substituents were able to influence the luminescence properties and yielded a direct correlation between the role of ligands and nanocluster properties. Thus, we conclude that structural variation in ligands with different substitution patterns away from the ligating atom is likely to yield nanoclusters with similar structure and composition, but they may still differ in terms of their optical properties. As the clusters are found to be a mixture of different compositions, along with the ligand structural variation, the cluster's composition pattern may also have an influence on the luminescence properties. Work is in progress to introduce more variation in the ligand (both substituent-wise and length-wise) to extract a structure–activity relationship between optical properties and ligand structure. We hope this study will encourage and guide other researchers in designing novel ligands to study the roles of ligands on the properties of metal nanoclusters. This, in turn, will help us in the synthesis of AuNCs with predictable properties.

Author contributions

A. K. B. conceived the presented idea and designed the experiments, and contributed to supervision, and analysis; B. B. and B. J. B. carried out the experiments and data analysis; A. W. carried out fluorescence experiments and data analysis; and A. S. and S. S. carried out MALDI experiments, T. G. A. experiments and data analysis. All authors contributed to writing the original manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Supplementary Material

RA-016-D6RA06384E-s001

Acknowledgments

B. B., B. J. B., A. W. and A. K. B. thank the ANRF, India, for financial support through the PAIR grant under the JNU-network (award no. ANRF/PAIR/2025/000029/PAIR). All authors thank Cotton University, Gauhati University, IIT Roorkee and IIT Guwahati for providing research facilities.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Additional data are available from the authors on request. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra06384e.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA06384E-s001
RA-016-D6RA06384E-s001.pdf (2MB, pdf)

Data Availability Statement

The data supporting this article have been included as part of the supplementary information (SI). Additional data are available from the authors on request. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra06384e.

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