## 1 Introduction  
Gold nanoclusters/nanoparticles have revolutionized nanomaterial-based sensors and biomedical technologies owing to their outstanding photoelectronic properties, such as superior plasmonic characteristics and chemical-biological inertness. In biomedical nanotechnology, surface-functionalized gold nanoclusters/nanoparticles capable of binding to numerous biomolecules, including antibodies, proteins, and oligonucleotides, have been developed for biomedical imaging, biosensors, and drug delivery [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. Recently, the plasmonic properties of gold nanoparticles have been utilized for the detection of cancer cell-derived exosomes [19, 20, 21]. In addition, the chemical inertness of gold nanoclusters/nanoparticles has driven the research of nanoparticle-mediated catalysis [22, 21, 23].  
Moreover, the variation in photo-absorption characteristics of gold nanoparticles with size is being studied to aid in the development of gold nanoparticle sensors [24; 25]. Furthermore, gold nanoclusters, which exhibit single-molecule-like behaviors owing to their extremely small size, have been widely studied as electron-trapping layers in photoelectronic sensors [26; 27]. The size of gold nanoparticles is considered to depend on various factors, including temperature, pH, and reaction time [28; 29].  
The Brust-Schiffrin method [30; 31], based on two-phase aqueous-organic medium and strong gold-alkanethiol ligand passivation, is a representative method for the synthesis of small-sized gold nanoparticles and nanoclusters (<5 nm) with a narrow size distribution. [PERSON] et al. first synthesized tiopronin-protected gold nanoclusters based on the Brust-Schiffrin method [32]. They synthesized tiopronin-protected gold nanoclusters in a methanol/acetic acid mixture (ratio of 6:1) and dispersed them in water through dialysis. As tiopronin has a carboxyl group at the active site that can bind with amines via the 1-ethyl-3-(3-dimethylamino)propyl carbodiimide/N-hydroxysuccinimide (EDC/NHS) reaction, tiopronin-capped gold nanoclusters/nanopriticles have been utilized in various biotechnologies. Most of the currently used tiopronin-capped gold nanoclusters/nanoparticles of sizes smaller than 5 nm have been synthesized using the method developed by [PERSON] et al. [33; 34; 35; 36]. To the best of our knowledge, the method developed by [PERSON] et al. is the only approach to synthesize tiopronin-capped gold nanoclusters/nanoparticles with sizes smaller than 5 nm reported to date. However, the synthesis method requires laborious and time-consuming dialysis processes to substitute the organic solvent and water.  
In this work, we present a novel and simple synthesis method to obtain tiopronin-capped gold nanoclusters/nanoparticles, with size control in the range from less than a nanometer to several nanometers. The advantages and highlights of this work are as follows. First, unlike the conventional Brust-Schiffrin method, the synthesis method proposed herein does not require dialysis processes because it uses water as a solvent during the entire synthesis. Second, our synthesis method uses sodium hydroxide to ensure the dispersion stability of the precursors, whereas the Brust-Schiffrin method uses organic solvents. Sodium hydroxide deprotonates the carboxyl groups in tiopronin and hinders drastic chemical reactions, resulting in stable nucleation and growth of the nanoclusters/nanoparticles. Finally, the boron byproduct was removed by converting it into volatile trimethyl borate using a small amount of methanol. The characteristics of the tiopronin-capped gold nanoclusters/nanoparticles with different sizes were investigated using Fourier-transformed infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), UV-Vis spectrophotometry, fluorescence spectrophotometry, dynamic light scattering (DLS), and X-ray diffraction (XRD).