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. 2026 Sep 14;11(38):57885–57894. doi: 10.1021/acsomega.6c07140

Facile Two-Step Size-Controlled Synthesis of Gold Nanoparticles Functionalized with Triethylamine-Cyclodextrins

Annisa Indah Reza †, Muhammad Eka Prastya ‡, Nagatoshi Nishiwaki §, Agustina Sus Andreani †,*
PMCID: PMC13629381  PMID: 42825145

Abstract

A two-step, size-controlled method is described for synthesizing uniform, quasi-spherical, and monodisperse gold nanoparticles (AuNPs) with diameters ranging from 15 to 22 nm, using a combination of triethylamine (TEA) and cyclodextrins (α, β, and γCD). The first step of sonication was conducted in a modified ultrasonic cleaner equipped with a dummy beaker to absorb excess energy from the adjacent transducer and a reaction beaker for the reduction process. Lower sonication energy settings (frequency = 37 kHz, P acoustic = 30%) enable more predictable and controlled surface plasmon resonance (SPR) in solution by avoiding the purple-to-blue color associated with the larger AuNP size. Hence, the emergence of a light-purple reaction mixture in the first step indicates the optimal time to transfer the solution to a water bath for subsequent heating in the second step. The resulting AuNPs typically exhibit a red wine color (λ = 515–525 nm). Among the systems evaluated, the ternary AuNPs-TEA-γCD system yielded the smallest nanoparticles, high stability, and noninclusion interactions between TEA and γCD, as evidenced by FT-IR and 2D-ROESY. More importantly, observation of AuNPs-TEA exhibited completely different SPR responses if subjected to only one step of the procedure. This procedure demonstrates high repeatability in producing smaller and more predictable aminated gold nanoparticles from the reactants used.

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1. Introduction

The Turkevich method, first developed in 1951, is a fast and widely used method to make colloidal gold nanoparticles in water. In this process, tetrachloroauric­(III) acid (HAuCl4) is reduced by using trisodium citrate (Na3Ct) as the reducing agent. Over time, many other reducing agents have been used to replace Na3Ct, allowing gold nanoparticles (AuNPs) to be made in a wider range of sizes and for more applications. − Several factors can significantly influence the effectiveness of the reducing agent during reactions, including the reactant concentration, solvent selection, pH, temperature, reaction time, and chosen protocol. Notably, even minor modifications in the technological process can produce substantial differences in the surface plasmon resonance (SPR) bands, which determine the unique visible properties of the resulting metal nanoparticles. These observations provide a fundamental rationale for investigating novel and efficient procedures for Au nanoparticle synthesis.

Amine groups are also recognized for their ability to reduce metal ions and to function as capping and stabilizing agents. − Numerous studies have investigated the synthesis of noble metal nanoparticles using a variety of amine structures, − amino acids, − and polymeric amines, − where the reaction is not limited to metal ion–nitrogen interactions. Particularly for tertiary amines such as triethylamine (Et3N, TEA), it was first recognized to promote nanoparticle nucleation by chelating Zn2+ during the fabrication of metal–organic frameworks (MOFs). Subsequent research has explored the use of TEA as a reducing agent in colloidal systems for the synthesis of Ag, − Mg, Zn, and bimetallic Au/Ag nanoparticles. However, despite this broader interest, to the best of our knowledge, only two published studies have ever addressed the application of TEA in the formation of single AuNPs over the past 20 years. , while its participation in ternary systems by combining with other reactants has remained unreported.

On the other hand, cyclodextrins (α, β, and γCD) are cage-like oligosaccharides that can act as effective reagents for the reduction of metal salts and bind to nanoparticle surfaces through chemisorption. They play a critical role in preventing nanoparticle aggregation, thereby enhancing solution stability. Previous studies have reported aminated cyclodextrins in ternary systems with metal nanoparticles, such as AgNPs-chloroethylamine-βCD, AgNPs-TEA-βCD with antibacterial activity, AuNPs-phenylalanine-βCD for photothermal drug release, and AuNPs with trimethylammonium-modified βCD (TMCD) for anticancer treatment.

Given this information, we intend to introduce a TEA-CD collaboration in AuNPs synthesis, with the innovation in the synthesis technique becoming the primary focus of this work. This is because the two mentioned protocols for AuNPs-TEA, utilizing electrochemical and conventional room-stirring methods, came with limited investigation into factors such as pH, concentration, and reaction time. , Meanwhile, the sonochemical method has recently emerged as a modest approach for nanoparticle generation, enabling precise control over particle size by adjusting ultrasonic parameters during synthesis. This technique allows for additive-free production, with particle size determined by specific instrument settings. Radicals generated through the sonolysis of H2O (H· and OH·) may induce the formation of secondary radicals from additives or surfactants (RH), in which secondary radicals subsequently diffuse into the bulk solution and participate in the following mechanism:

RH+OH·(H·)→R·+H2O(H2) 1
Au3++reducing species⁡(R·,OH·,H·,etc.)→Au0 2

Although the sonochemical reduction of Au ions has been extensively investigated using horn-type or tip-type sonicators, − a standard ultrasonic cleaner commonly available in laboratories has also achieved this purpose. However, no studies have systematically controlled the sonochemical process for AuNP synthesis using an ultrasonic cleaner. Therefore, this work aims to determine the optimal conditionsincluding frequency, P acoustic, cavitation mode, mixture volume, and nanoseed growth timethat yield the desired nanoparticle size, as determined by surface plasmon resonance (SPR) color monitoring of colloidal AuNPs. The second step in this work, water-bath heating, was shown to accelerate and terminate the reduction process following sonochemical treatment. This approach also produces distinct results when each step is executed separately while using the involved reactants.

2. Experimental Section

2.1. Materials and Instruments

A gold source (HAuCl4·3H2O, ≥99%), triethylamine (TEA, ≥99%), N-methyl-2-pyrrolidone (NMP, ≥99%), pyrrole (P, ≥99%), pyrrolidine (PY, ≥ 99%), ethylamine (EA, 66.0–72.0%), sodium hydroxide (NaOH), and hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich and utilized without further purification. Cyclodextrins (α, β, and γCD, ≥99%) were purchased from Glentham Life Sciences. Water used throughout (including solution preparation and rinsing) was purified by a Millipore system (resistivity 18.2 MΩ·cm) using a Direct-Q 3 UV water purification system (Merck). An ultrasonic cleaner (ELMA Elmasonic P, 37/80 kHz, P acoustic 30–100%, capacity 2.75 L), digital water bath DWB 2H 6L (Prio), digital analytical balance, laboratory glassware (Pyrex), micropipette (Thermo Scientific), silicon wafer, and wire were also used.

2.2. Synthesis of AuNPs

AuNPs were synthesized in the following two types of mixtures. The first mixture involves a single TEA application to produce AuNPs-TEA (mixture A), whereas the second introduces both TEA and CD simultaneously (mixture B), resulting in AuNPs-TEA-CD, as illustrated in Figure .

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Conceptual plan of AuNP synthesis and comparison of cyclodextrin structure.

Stock solutions of HAuCl4 (10 mM), TEA (0.1 M, pH 9.0–12.0), and all CDs (0.1 M, pH 11.0) were prepared. Other amine derivatives were also prepared at 0.1 M, and their pH was measured immediately after preparation without further adjustment. The two-step process, consisting of sonochemical treatment followed by water-bath heating, is shown in Figure A, while modifications to the ultrasonic cleaner are presented in Figure B. Optimization of mixture A involved varying ultrasonic cleaner modes (degas, sweep, and pulse), reaction volumes, monitoring intervals for AuNPs-TEA formation, and water bath heating conditions. After confirming the significance of the two-step method, the protocol was further assessed for other amine derivatives, the pH of TEA, the [TEA/AuCl4 –] ratio, and the [TEA/CD/AuCl4 –] ratio to generate AuNPs-TEA-CD. Sonication was performed at 37 kHz with a P acoustic value of 30% for 2 min. The reaction beaker was then allowed to stand for 1–3 min, during which a light-purple color developed, indicating the initiation of AuNP seed formation. Once the color appeared, the mixture was transferred to a test tube and heated in a water bath at 90 °C for 1 h, resulting in a transparent red-wine color. For mixture B, reactants were combined directly at the optimized molar ratio and subjected to the same procedure. The method was repeated 10 times to confirm repeatability for all AuNPs. Periodic stability testing of AuNPs stored at room temperature was conducted using UV–vis spectroscopy.

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(A) Conceptual diagram of AuNP synthesis with results under different variation procedures. (B) Visual image and illustration of the modification on the ultrasonic cleaner’s interior.

2.3. Characterization of AuNPs

UV–visible (UV–vis) spectra were obtained using a Shimadzu S-600 spectrophotometer within the 400–800 nm range. The particle morphology, size, and phase composition were determined by high-resolution transmission electron microscopy (HRTEM, Talos F200X), and its crystalline nature was assessed by selected-area electron diffraction (SAED). Energy-dispersive X-ray analysis (EDAX) was conducted with a Talos scanning transmission electron microscope (STEM) equipped with a high-angle annular dark-field (HAADF) detector. TEM images were analyzed using ImageJ 1.54g (Wayne Rasband, National Institutes of Health, USA). The 1H NMR and 2D-ROESY spectra were recorded using a Bruker Advance Neo 700 MHz spectrometer with D2O as the solvent. The hydrodynamic diameters and zeta potential were measured using a Horiba SZ-100. Fourier transform-infrared (FT-IR) spectra (Shimadzu Prestige-21) were recorded using the silicon wafer method over a range of 400–4000 cm–1.

3. Results and Discussion

3.1. Synthesis of AuNPs

A dummy beaker and a reaction beaker were positioned above each available transducer in the ultrasonic cleaner. Both beakers were securely fixed and centered over the transducer area with a circling wire (Figure B). The dummy beaker absorbed excess energy from the neighboring transducer, preventing unintended energy transfer to the reaction beaker that could affect the reaction outcome. Deviating from this designfor example, using only one reaction beaker or placing two reaction beakers for simultaneous reactionsresulted in a purple or blue solution. These colors are typically associated with larger gold nanoparticles at red-shifted wavelengths (larger wavelength values), providing a visual indicator of particle size that was later found to be significant for reaction monitoring. Additionally, omitting the circling wire allowed the beakers to move during the reaction, which reduced control over the energy delivered to the reaction beaker and produced variable SPR colors of AuNPs-TEA (Figure S1).

The minimum settings of the ultrasonic instrument, specifically a frequency of 37 kHz and P acoustic at 30%, were selected because higher available settings (80 kHz and 100% P acoustic) produced a dark-purple color in the mixture within 1 min of reaction. Sonochemical synthesis, especially at low ultrasonic frequencies, is preferred because it avoids unnecessary complications and residues, whereas ultrasonic irradiation power determines the chemical intermediate generation at pressure–temperature relationships during the redox reaction. The 2 min sonication period was chosen to ensure repeatability with no noticeable rise in temperature at the ultrasonic water bath, as a 1 min duration often failed to induce a color change, even 24 h postsonication. Sonication for more than 2 min consistently produced an immediate purple-to-blue transition.

The initial evaluation examined variations in cavitation-delivery modes, specifically degas, sweep, and pulse (Figure A), using [TEA/AuCl4 –] = 20/1. Each mode in ultrasonic cleaners has a distinct function, enhancing cleaning efficiency through different cavitation delivery mechanisms. Pulse mode generates continuous, high-power bursts from the transducer, resulting in a sharp and clean signal on the UV–vis spectrum. On the contrary, degas mode alternates between “on and off” bursts, while sweep mode distributes cavitation more broadly. After selecting pulse mode, the reaction volume in the beaker was varied to 10, 15, and 20 mL while maintaining a constant molar ratio as in mode selection. Although the solutions exhibited similar SPR peaks, indicating procedural consistency, more dilute solutions required longer nucleation times and produced lower absorbance values (Figure B). Therefore, a reaction volume of 10 mL was optimal for this experimental setup. It is necessary to note that the UV–vis spectra for the first two observations above were recorded after the solution was left for 24 h following sonication, to allow the development of the red-wine color. Upon remeasurement, the absorbance of the 10 mL mixture from Figure B had increased after 48 and 72 h (Figure C). This outcome suggested that the nucleation process had been ongoing before reaching the optimum point.

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UV–vis spectral of AuNP synthesis for variations in (A) modes and (B) solution volume. (C) Increasing absorbance over days. (D) Comparison of heating treatment. (E) Variation of amine derivatives. (F) pH optimization of TEA. (G) Molar ratio of [TEA/AuCl4 –]. (H) Molar ratio of [TEA/CD/AuCl4 –]. (I–L) Repeatability in ten replications to produce AuNPs-TEA, AuNPs-TEA-αCD, AuNPs-TEA-βCD, and AuNPs-TEA-γCD, respectively. (M) Stability monitoring over three months. (N) Plotted graph of particle hydrodynamic size (bars) and zeta potential (dots) of AuNPs.

Following these results, a second procedure was implemented to accelerate reduction and halt nanoparticle growth using water-bath heating. When a light-purple color appeared in the reaction beaker, typically within 1 to 3 min after sonication (see Figure S2), the solution was immediately transferred to a test tube and heated in a water bath at 90 °C for 1 h. This process resulted in increased absorbance (Figure D). When AuNPs-TEA synthesis was performed solely by water-bath heating at the same or higher temperatures, a bichromatic effect appeared, with two distinct colors depending on the direction of incoming light (as in Figure A from mixture A).

Evaluation of alternative amine derivatives and water as a control did not result in satisfactory AuNP formation (Figure E). Although certain amines, including NMP, PY, and EA, provided conditions comparable to those for TEA, the method remains effective only for TEA. Increased basicity in the reaction mixture leads to higher absorbance (Figure F). However, the optimal pH was determined to be 11.0, as pH 12.0 introduced noise absorption at lower wavelengths and faster agglomeration after 1 day. Molar optimization of [TEA/AuCl4 –] gave 20/1 as the optimum yield, where a higher concentration of TEA led to a red shift on SPR (Figure G). On the other hand, when varying the molar ratio of [TEA/CD/AuCl4 –] in the ternary system, both α and γCD systems gave 20/10/1 as the optimum value, whereas βCD demanded a higher ratio of 20/15/1 (Figure H). This could be due to the fact that the low solubility of βCD has already affected its concentration in the stock solution. Hence, increasing the volume of the βCD in the solution mixture was the way to increase absorbance. However, excess cyclodextrin molecules may bridge multiple particles, increase solution viscosity, and lead to particle clumping and reduced stability, ultimately selecting a molar ratio of 20/10/1 as the optimum for all ternary AuNP systems.

Eventually, ten subsequent syntheses were carried out to produce all AuNPs solution, where the method’s good repeatability and precision were confirmed by UV–vis measurements (Figure I–L). As seen, all AuNPs-TEA-CDs exhibited higher absorbance than AuNPs-TEA due to the presence of dual active reactants in the system. Interestingly, an additional evaluation of the two-step method, in which the mixture contained only AuCl4 – and CD, did not yield successful results, highlighting the distinct outcomes of this procedure (see Figure S3). In contrast, using only water bath heating to generate AuNPs-CD was successful, whereas elevated temperatures are conducive to CD performance, as reported in several studies. −

3.2. Stability Test of AuNPs

The periodic stability test demonstrated that AuNPs-TEA exhibited the most pronounced decrease in absorbance compared to the other AuNPs-TEA-CD samples (Figure M). This decrease is attributed to the formation of thread-like structures, where self-organized nanoparticles adhere to a glue-like substance (see Figure S4). In the case of AuNPs-TEA-βCD, the observed low stability over time is likely due to intramolecular hydrogen bonding or a tendency for CD self-aggregation. Eventually, AuNPs-TEA-γCD displayed the highest stability, which is supported by its larger hydrodynamic particle size (66.2 ± 2.1 nm) that likely offers steric stabilization, and its zeta potential value (−57.7 ± 0.2 mV), which is more negative than those of the other samples and may provide electrostatic repulsion, thereby maintaining high stability (Figure N). The negative surface charge observed on all AuNPs is consistent with the pH setting used for TEA and CD.

3.3. Characterization of AuNPs

3.3.1. Surface Characterization

TEM analysis demonstrated that all nanoparticle samples exhibited a uniform spherical morphology (Figure A–D), with the SAED pattern confirming the typical d spacing of Au (see Figure S5). Histograms derived from the TEM images illustrate the average size and size distribution of AuNPs (Figure E–H). Among the samples, AuNPs-TEA displayed the largest particle size and the broadest distribution (10–40 nm), whereas AuNPs-TEA-γCD exhibited the smallest size and a narrower distribution (5–25 nm). The particle size of AuNPs-TEA-βCD was slightly smaller than that of AuNPs-TEA-αCD. These findings accumulatively suggest that the ternary system of AuNPs-TEA-γCD provided an excellent reducing and stabilizing environment, as evidenced by the smaller particle size and longer stability. After one month of aging at room temperature, TEM images of all AuNPs revealed significant clustering, although the spherical morphology was retained (Figure I–L). Noticeably, the enlarged size of colloidal AuNPs-TEA is displayed in Figure M and N. Once these types of color appear after prolonged sonication or an uncontrolled sonochemical process, they cannot be transformed to a lower-wavelength state by performing the second step of water-bath heating. Consequently, by noting the color associated with larger AuNPs-TEA, predicting and controlling the desired AuNP size using the proposed method is achievable.

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(A–D) TEM images of AuNPs in the first month and (E–H) particle size distribution of AuNPs-TEA, AuNPs-TEA-αCD, AuNPs-TEA-βCD, and AuNPs-TEA-γCD, respectively. (I–L) Transformation of TEM images of AuNP species after aging for one month at room temperature. (M, N) TEM images of AuNPs-TEA in purple and blue color.

3.3.2. Binding Interactions

At first, HAADF-STEM images of comparable AuNPs regions provide insight into surface coverage. As shown in Figure A–D, nitrogen is concentrated on the gold surface, while oxygen from CD is more diffusely distributed around the nanoparticles. This indicates that oxygen is dispersed throughout the particle matrix and acts as a support for nano Au. Previous reports on molecular dynamics simulations have compared the adsorption profiles of α, β, and γCD on AuNPs through complex interactions with multiple binding modes. − However, distortions and twisting of the macrocycle occur only in γCD due to its larger size. Therefore, γCD might have a greater stabilizing effect than the other CDs. These computational results may explain the higher stability observed for AuNPs-TEA-γCD in this work. Still, further clarification is obtained through FT-IR and 1H NMR spectroscopy analyses.

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(A–D) HAADF-STEM individual and overlay element images of AuNPs-TEA, AuNPs-TEA-αCD, AuNPs-TEA-βCD, and AuNPs-TEA-γCD, respectively. The elemental mapping of individual Au, N, C, and O elements confirming their core distinct distribution in the bulk quantity of AuNPs.

FT-IR spectroscopy can identify the alteration of original functional groups of each reactant (Figure A) corresponding to their AuNPs (Figure B). The fingerprint signal for TEA is solely noticed by C–N stretching at 1045 cm–1. Meanwhile, several signals related to CD can be assigned to C–O–C stretching at lower wavenumbers (1013–1056 cm–1), C–H stretching (2972–2926 cm–1), and O–H stretching at higher wavenumbers (3727–3313 cm–1), in which there is a distinctly wider O–H stretching for γCD compared to αCD and βCD due to its higher solubility and larger structure that provide more hydrogen-bonding flexibility. The C–N stretching signal of AuNPs-TEA shifts significantly to 1164 cm–1 and becomes narrower, suggesting interactions with the gold surface. A minor overlap between O–H and N–H stretching absorptions is observed as a lower intensity at 3383 cm–1, which may result from degradation during the heating process that converts TEA to primary or secondary amines or the protonated TEA forms. For AuNPs-TEA-CD, C–O–C and C–N stretching vibrations appear in close proximity at lower wavenumbers, resulting in a completely overlapping signal for AuNPs-TEA-βCD (1073 cm–1), but these signals remain distinguishable for AuNPs-TEA-αCD (1144 cm–1) and AuNPs-TEA-γCD (1150 cm–1). Furthermore, a characteristic CO band as a carboxylate, which could indicate the formation of an oxidation product of CD during the reduction of AuCl4 – to Au0, is observed across the ternary mixtures. However, the respective signals are prominent for AuNPs-TEA-αCD (1643 cm–1) and AuNPs-TEA-βCD (1640 cm–1) but less pronounced for AuNPs-TEA-γCD. Finally, stretching vibrations of O–H and N–H resulted in broad bands assigned for AuNPs-TEA-αCD (3347 cm–1), AuNPs-TEA-βCD (3389 cm–1), and AuNPs-TEA-γCD (3337 cm–1).

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Normalized FT-IR spectra to evaluate transformation of functional group in (A) the initial reactant and (B) after producing AuNPs. (C) Plotted area from subtracted absorbance using Gaussian on respective bands to calculate the carbonyl index (CI) (for details, see Figure S7). (D) Calculated CI values for each CD from FT-IR analysis.

The calculation of the carbonyl index (CI) by comparing the ratio of the integrated area between the CO peak to the aliphatic C–H peak of CD (treated as reference peak) is useful to quantify the degree of oxidation following eq .

CI=Area under band of COArea under band of C−H 3

By using Gaussian to plot the area under the absorbance curve against wavenumber (Figure C), the rate of CD in generating carboxylate as an oxidized product can be rationalized, thereby indicating the involvement of CD as a reducing or stabilizing agent in the system. As the order from the lowest to highest CI appears as CIγ < CIα < CIβ, γCD exhibits the lowest rate of carboxylate generation and is more likely to function as a stabilizing agent than other CDs (Figure D).

To better understand the binding interactions, AuNPs-TEA-γCD was subjected to nuclear magnetic resonance (NMR) analysis. 1H NMR (see Figures S8 and S9) was employed to investigate the changes in the chemical shifts of free TEA and CD protons upon incorporation into the AuNP framework. As shown in Figure A, the 1H NMR peaks of free TEA (Ha at 2.53 ppm and Hb at 0.99 ppm) shifted significantly downfield in both AuNPs-TEA and AuNPs-TEA-γCD. The peaks reappeared at approximately 3.06 (Ha) and 1.26 ppm (Hb), respectively. This observation suggests that the transfer of electron density from the nitrogen atom to the gold surface plays a critical role in the reduction process. On the other hand, the sharp signals of free γCD at 5.03, 3.85, and 3.81–3.77 ppm were assigned to H1, H3, H5, H6, and H6′, respectively. The H2 proton appeared at 3.58–3.56 ppm as a doublet of doublets, while H4 was observed at 3.51 ppm as a triplet. In the ternary AuNPs, the original γCD proton signals exhibited only minor downfield shifts, which once again proves that γCD is less involved in the reduction of AuCl4 – than TEA. Interestingly, the H1 proton signal was doubled by introducing H1′ (complexed state) at 5.10 ppm. This phenomenon has been attributed to the slow exchange regime observed in NMR studies of cyclodextrin-based host–guest assemblies. This complexation may reflect either an inclusion interaction, in which the aliphatic chain of TEA enters the cavity of γCD, or a noninclusion interaction.

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(A) Comparison 1H NMR spectra of TEA, γCD, AuNPS-TEA, and AuNPS-TEA-γCD. (B) 2D-ROESY of AuNPS-TEA-γCD showing no cross-peaks between TEA and γCD. (C) The proposed interaction between TEA and γCD occurs as noninclusion.

A 2D-ROESY analysis was conducted to investigate the complexation between TEA and γCD. Figure B shows that no cross-peak was observed between TEA and γCD, which indicates the absence of an inclusion interaction. The only cross-peak detected was between the inner-ring protons of γCD, which could be derived from hydrogen bonding from the hydroxyl group. These findings emphasize that γCD functions as a supporting agent that stabilizes AuNPs, which are readily available in dispersion-free form, or interacts with TEA via a noninclusion mechanism (Figure C). Hence, this conclusion aligns with the results from HAADF and FT-IR analyses.

4. Conclusions

A facile, modified method for synthesizing functionalized gold nanoparticles (AuNPs) was developed using triethylamine (TEA) and cyclodextrins (CDs) via a two-step process integrating modified sonication and water-bath heating. In this method, TEA serves as the reducing agent while cyclodextrins function as stabilizing agents. Distinct surface plasmon resonance (SPR) signals were observed, which could be visually monitored as indicators of the formation of smaller AuNPs, either as AuNPs-TEA or AuNPs-TEA-CD. Following the initial sonication step, an extended set aside period produced a more intense purple color, signifying the formation of larger nanoparticles. Therefore, postponing the water bath heating step after sonication results in a relatively larger SPR wavelength (purple or blue color) and hinders the shift toward the blue-shifted region.

To precisely control the emergence of the light-purple indicator in the reaction mixture, modification of the sonication instrument with a water bath is essential. Under this protocol, AuNPs-TEA-CD exhibit a uniform spherical dispersion, smaller size, and greater stability than AuNPs-TEA alone. Notably, the same protocol did not yield AuNPs-CD, even after multiple trials with fresh reactants. The formation of AuNPs under the proposed method was confirmed by UV–vis and FT-IR spectroscopy. HAADF mapping and profiling revealed that an amine-rich surface was consistently observed on all synthesized AuNPs, unlike cyclodextrins. Moreover, γCD was found to be more effective at stabilizing Au0 than other cyclodextrins due to its larger cavity, which promotes stronger and more complex molecular interactions. This claim is further supported by FT-IR, which shows the lowest carbonyl index, and by 1H NMR, which shows a lower degree of downfield shifting profile. Although 2D-ROESY was conducted only for AuNPs-TEA-γCD due to its unique profile across the characterizations and limited employment compared to the other CDs, this work offers a simple and novel modification of an ultrasonic cleaner for AuNPs. Variations in pH, reducing agents (not limited to amines), concentration, and temperature warrant further investigation, as these factors may facilitate the development of nanomaterials with unique properties by tuning their plasmon resonances. Finally, applications of AuNPs obtained in this work, such as colorimetric sensing or biosensing, are currently being developed.

Supplementary Material

ao6c07140_si_001.pdf (1.6MB, pdf)

Acknowledgments

This work was supported by the prototype of Underwater Detector Network Research and Innovation Results for Fiscal, from Research Organization for Electronics and Informatics, year 2026 (19/III.6/HK/2026). The authors would also like to acknowledge the facilities and scientific and technical support from Advanced Characterization Laboratories Serpong, National Research and Innovation Agency (BRIN) through E-Layanan Sains, Badan Riset dan Inovasi Nasional (ELSA). BioRender, Chemdraw, and Origin were used to create images and graphs. The Article Processing Charge for the publication of this research was funded by the Kochi University of Technology, Japan.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c07140.

  • Additional details on the experimental procedure, including a photograph of colloidal AuNPs in various SPR colors; TEM images; SAED patterns; histograms with the Gaussian plotted; and 1H NMR spectra (Figures S1–S9) (PDF)

A.I.R.: Conceptualization, data curation, and writing the manuscript. N.N.: Provision of valuable corrections and suggestions. M.E.P. and A.S.A.: Supervision and funding acquisition for the project.

The authors declare no competing financial interest.

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