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Article

On the Importance of Fresh Stock Solutions for Surfactant-Free Colloidal Syntheses of Gold Nanoparticles in Alkaline Alcohol and Water Mixtures

by
Jonathan Quinson
Biochemical and Chemical Engineering Department, Aarhus University, Åbogade 40, 8200 Aarhus, Denmark
Inorganics 2023, 11(4), 140; https://doi.org/10.3390/inorganics11040140
Submission received: 16 February 2023 / Revised: 21 March 2023 / Accepted: 22 March 2023 / Published: 25 March 2023
(This article belongs to the Section Organometallic Chemistry)
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Abstract

:
A room temperature surfactant-free synthesis of gold nanoparticles in the size range 10–20 nm that only requires HAuCl4 as the precursor, NaOH as the base, water as the solvent and a mono-alcohol such as methanol or ethanol as the reducing agent, has recently been detailed. This approach is promisingly simple to obtain colloids stable for months. Here, it is shown that the use of fresh stock solutions of base is one key to ensure the formation of stable surfactant-free small-sized gold nanoparticles. The need for relatively freshly prepared stock solutions of base does not appear to be as crucial for syntheses using stabilizers and/or viscous solvents such as glycerol. The possibly overlooked importance of the age of the stock solution of base might account for the limited interest to date for the simple room temperature synthesis in low viscosity mono-alcohols highlighted.

1. Introduction

Gold (Au) nanomaterials (NMs) and nanoparticles (NPs) combine unique surface plasmon resonance (spr) properties, biocompatibility and catalytic activity that make them relevant for a range of applications in medicine, sensing or energy conversion, to name only a few examples [1,2,3]. Detailed studies of the properties of Au NPs rely on the careful synthesis of the Au NMs. While numerous synthetic approaches have been reported [4], state-of-the-art colloidal syntheses remain the Turkevich–Frens [5,6] and the Brust–Schiffrin methods [7,8]. An ultimate synthesis complying with the principles of Green Chemistry for NM synthesis [3,9,10,11,12] would however be surfactant-free [13], performed at room temperature (RT) and would require only relatively few and safe chemicals such as simple alcohols and water [10,11].
We recently reported a synthesis that combines all these desirable features using ethanol (EtOH) as a reducing agent in aqueous alkaline solutions [14]. Via a detailed parametric study, we could identify that 30 v.% EtOH and a Base/HAuCl4 molar ratio around 4 are optimized conditions to obtain ca. 10 nm stable colloidal Au NPs using 0.5 mM HAuCl4. These conditions were not identified in earlier work [15]. The simplicity of the method leaves us to wonder why it has not been reported and exploited before.
In our previous work, we showed and commented that a careful and explicit report on the order of addition of the chemicals is important [14]. In addition, we can stress that the synthesis conditions in a range of RT syntheses are not necessarily detailed enough to be fully reproducible; for instance, using glycerol as reducing agent. For such viscous solvents it is often not entirely clear how long the reactions are left to proceed and/or how stirring was performed, if any, and for how long [16,17]. These examples are not meant to negatively point out the work of the authors that remains extremely valuable, but rather to illustrate general challenges. Reproducibility is indeed an acknowledged issue in experimental sciences and the synthesis of Au NPs is not an exception. Liz-Marzán et al. summarized various challenges in the preparation of Au NMs and listed various ‘tricks’ to best achieve controlled syntheses [18,19]. Impurities, e.g., from additives such as surfactants or from the water/solvents, can be a source of irreproducibility for the synthesis [20], which can ultimately affect the resulting properties of the NMs [18,19,21].
The difficulty to report fully comprehensive protocols, and in particular in the chemical and material sciences, can be explained by several factors: (i) platforms and publication formats to report in length detailed protocols and the related results are often not available or not favoured; (ii) it is often not known to the experimentalists what really matters at the time of the first reports, and (iii) detailing negative results (i.e., what does not lead for instance to Au NPs) is not as valued as positive results [22]. Initiatives are developed to address these challenges with for instance new publishing formats [23]. Furthermore, the input of data-driven science, for which positive and negative results are equally valuable to train algorithms for machine learning and artificial intelligence, is also calling for more reports on negative results [24]. Finally, ensuring that ‘tricks’, and parameters to pay attention to, are shared broadly, will ensure more widely implementable protocols in a more timely, cost-effective and time-effective manner.
In agreement with these general principles, we document in this report the importance of using freshly prepared stock solutions of base to ensure more reproducibility in the RT surfactant-free syntheses of colloidal Au NPs in mono-alcohols and water mixtures. We also give a possible explanation on why the effect of aged stock solutions of base was not stressed earlier and how this could account for the fact that surfactant-free syntheses of Au NPs in low boiling point solvents were not exploited further.

2. Results

2.1. Overview

The Au NPs are here synthesized using NaOH, water (H2O), ethanol (EtOH) or methanol (MeOH)—where the last two chemicals are here jointly referred to as mono-alcohols (ROH)—and HAuCl4 was used as the Au precursor. A schematic representation of the synthesis is given in Figure 1. The synthesis is further detailed in the Materials and Methods section. In addition, various metrics retrieved from UV-vis measurements used below are also detailed in the Materials and Methods section. HAuCl4 is reduced at RT by the ROH. Stable colloidal NPs are obtained, despite the absence of surfactants/ligands or capping agents [14]. While the synthesis is reproducible [14], we sometimes experienced challenges to replicate some results. In agreement with what we previously observed to obtain stable ca. 10 nm surfactant-free Au NPs, an optimal Base/Au molar ratio is around 4 [14]. However, we noticed during the present study that this optimal ratio can be slightly lower or slightly higher depending on the stock solution of base in water, probably due to slightly different concentrations achieved for the different stock solutions.
Another source of irreproducibility with a stronger effect on the NP properties identified and documented here is the age of the stock solution of the base. Various experiments were performed for which an account and further details are given below and in Supplementary Materials. We here compare syntheses performed using a fresh stock solution of NaOH in water stored in glass or polypropylene (PP) containers versus stock solutions several months (>1 month) old stored in glass (Pyrex®) containers. As it is discussed below, fresher solutions are to be preferred, and avoiding glass is to be preferred. The cases of fresh solutions stored in plastic containers, referred to as ‘fresh’ solutions below, and old solutions stored in glass containers, referred to as ‘old’ solutions below, are therefore two extreme scenarios used here as illustrative examples.

2.2. Effects of the Age of the Stock Solutions of Base

2.2.1. Using EtOH as Reducing Agent

We previously showed that low ROH content (<30 v.%) are to be preferred if EtOH is used to obtain small size ca. 10 nm Au NPs [14]. Using 10 v.% or 30 v.% EtOH and a fresh (<1 week) stock solution of NaOH, leads to small size NPs ca. 9 nm, see TEM in Figure 2a and results in Table 1. When 10 v.% EtOH is used, the size distribution is slightly larger, and this can be explained by synthetic conditions where there is not enough reducing agent (EtOH) which leads to poorly controlled syntheses of Au NPs. Nevertheless, these NPs prepared using fresh stock solutions of base are characterized by a well-defined plasmon resonance around 530 nm for 10 v.% EtOH and 517 nm for 30 v.% EtOH. In contrast, not NPs but rather large aggregates are obtained with an aged stock solution of NaOH, see Figure 2b, characterized by a maximum of absorption in UV-vis measurements above 540 nm with poorly defined features (larger Δλ/λspr values above 15% when an old stock solution is used versus ca. 6–7% when a fresh stock solution of base is used), Figure 2c. As a result of these features, other indicators such as A650/Aspr have lower values using a fresh stock solution of base (<0.5) compared to the cases where an old stock solution of base is used (>0.5). Alternatively, A380/A800 values are higher (>8) using a fresh stock solution of base versus the case where an old stock solution of base was used (<2) as indicated in Supplementary Materials. Furthermore, the relative intensities at 400 nm are higher using fresh stock solution of base for a given EtOH content.

2.2.2. Using MeOH as Reducing Agent

The effect of using a fresh or an old stock solution of base was then investigated using MeOH as a reducing agent at different v.% as illustrated in Figure 2d–f. Using MeOH, the synthesis is more robust to various synthetic parameters and leads to larger NPs than when EtOH is used [14]. Therefore, in this case, 10 v.%, 30 v.% and 50 v.% MeOH were investigated, and an overview of the results is given in Table 1. Here, as well, at lower amounts of MeOH, using a fresh stock solution of base leads to poorly defined NMs with a network structure, to be related to the little amount of reducing agent available. Using an old stock solution of base did not lead to stable NM colloids either for 10 v.% MeOH. For 30 or 50 v.% MeOH contents, NPs ca. 20 nm in diameter are obtained using freshly prepared stock solutions of NaOH, whereas larger nanostructures or even networks are obtained using an old NaOH solution. UV-vis spectra obtained using a fresh stock solution of NaOH show a better-defined localized spr with a maximum absorbance at 531–542 nm (with a higher wavelength observed for higher MeOH contents) and relatively well-defined peaks (Δλ/λspr values around 7–9%), compared to the case where an old stock solution of base was used where the resulting samples are characterized by a maximum absorption above 540 nm, see Figure 2f and large Δλ/λspr values (above 15%). Furthermore, as a result of these features, other indicators such as A650/Aspr have lower values when a fresh stock solution of base is used compared to the case where an old stock solution is used (<0.5 versus >0.7, respectively). Alternatively, A380/A800 values are higher using a fresh stock solution of base compared to the case where an old stock solution of base is used (>4 versus <2, respectively), as indicated in the Supplementary Materials. Furthermore, the relative intensity at 400 nm is higher using fresh stock solution of base for a given MeOH content.

2.2.3. Synthesis in Presence of Additives

Finally, we also investigated the effect of an old NaOH stock solution using different common additives for the synthesis of Au NMs such as lithium tricitrate (Li3Ct), sodium tricitrate (Na3Ct) and polyvinylpyrrolidone (PVP), as per our previous work with freshly prepared stock solutions using 30 v.% EtOH or MeOH [14], see Figure 3 and Table S2. As opposed to the cases of surfactant-free syntheses, the presence of additives does not affect much the general shape of the UV-vis spectra where poorly defined plasmon resonance peaks above 525 nm are observed regardless of the use of fresh or old stock solutions of base. The A400 values are even higher when an old stock solution of base is used. The exception is when MeOH is used and where the use of citrate-based additives and fresh stock solution of base leads to NMs with a better defined spr characterized by λspr values below 530 nm and where slightly higher A400 values are obtained using a fresh stock solution of base.

3. Discussion

From the characterization reported in Figure 2 and Table 1, it is clear that using an old stock solution of NaOH is detrimental to obtain small-sized NPs using EtOH or MeOH as reducing agents. The λspr values, the related Δλ/λspr values are all relatively larger when old stock solutions of base are used for different ROH contents. Considering that a higher absorption at 400 nm is an indicator of a higher conversion for HAuCl4 to NPs [25], the UV-vis measurements also suggest that a better yield is achieved using fresh stock solutions of NaOH. Using lower A650/Aspr values as a proxy for more stable NPs (or higher A380/A800 values reported in Supplementary Materials), the colloids prepared using fresh stock solutions of base are expected to be more stable. These results are confirmed by TEM, Figure 2, where NMs prepared using fresh stock solutions of base lead to relatively small spherical NPs, Figure 2a,d, whereas using old stock solutions lead to larger network-like structures with a characteristic size for 10 s of NMs for the smallest dimensions but that overall appear as bulkier NMs of 100 s of nanometers, Figure 2b,e.
To assess the general effect(s) of using fresh or old stock solutions of base, we considered other surfactant-free syntheses. Glycerol has been a preferred solvent and reducing agent to date for the RT synthesis of Au NPs [26]. Despite the fact that surfactant-free syntheses are possible using glycerol [16], the presence of PVP is often preferred [27]. The effect of using fresh or aged stock solutions of base with glycerol as a reducing agent is investigated here. In this case, as well, larger NPs are obtained using aged stock solutions of base compared to the case using a fresh stock solution of base, as illustrated in Figure 4, see also Table S3. However, the effect of using aged stock solutions of base on the resulting Au NPs is not as pronounced for glycerol. Considering the absorbance at 400 nm as an indicator, in this case, the yield is even slightly higher using an old stock solution of base. The λspr values are 526 and 528 nm, with respective Δλ/λspr values of 7.2 and 6.1%, for colloidal NPs obtained using a fresh and an old stock solution of base, respectively. The NPs are even slightly more stable using an old stock solution (A650/Aspr values of 0.07 versus 0.14 when a fresh stock solution is used, A380/A800 values of 46.7 versus 17.8 when an old or fresh stock solutions of base are used, respectively). Nevertheless, the NPs are slightly smaller (10.1 ± 2.7 nm) for NPs obtained using a fresh stock solution versus an old stock solution (17.3 ± 4.3 nm). This robustness of the synthesis using glycerol might account for the fact that glycerol has been more widely studied to date as a reducing agent than other polyols or ROH. However, the high viscosity of glycerol is a major challenge for further simple application of the Au NPs [14]. Furthermore, using an old stock solution of NaOH and ethylene glycol (EG) as an alternative polyol and reducing agent less viscous than glycerol did not lead to NPs, see Table S2 and Figure S1. These results overall justify the preference stressed here for relatively fresh stock solutions of NaOH for surfactant-free Au NP synthesis in low boiling point solvents.
To test even further the general effect(s) of using a fresh or an old stock solution of base, we finally considered surfactant-assisted syntheses. Larger NPs were obtained in presence of additives compared to a case without additives [14]. This is explained because the additives selected can also play the role of reducing agents. There is therefore more and different reducing agents in solutions. Using a fresh or an aged stock solution of base in these cases did not affect the results as much, Figure 3. Slightly more defined structures and smaller size NPs were observed by TEM when a fresh stock solution was used. Comparable yields or even higher yields (based on the absorption at 400 nm) were obtained using an aged stock solution of NaOH. The dependence of the results to the age of the stock solutions seems to be screened when additives such as stabilizers are used, which can explain why the effect of a fresh versus an old stock solution was not necessarily flagged as an important experimental parameter. A noticeable exception pointing towards the benefits of fresh stock solutions was for MeOH and Li3Ct or Na3Ct, where a clear plasmon resonance, indicative well-defined small size spherical NPs, was observed using a fresh stock solution of base. This observation further stresses the complex interplay between precursor, additives and solvents/reducing agents [14].
Although some ageing is necessarily occurring for most stock solutions, it remains a parameter seldom investigated and difficult to rationalize because it is challenging to fully control. The fundamental reason for the strong differences observed here between the fresh and aged stock solutions is not clear at this stage. Various parameters might change over time such as pH, or presence of impurities and/or side reactions, not to mention different interactions of the precursors with the species present in solution before, during and after reaction. For instance, glass corrosion is likely to take place despite the relatively low NaOH concentration [28,29]. The importance of using fresh solutions of NaOH is probably an overlooked parameter because it is more pronounced in surfactant-free approaches in low viscosity solvents than in approaches using additives or viscous solvents, which can explain why, although some reports considered an approach similar to the one detailed here [15], the general concept of surfactant-free colloidal syntheses in EtOH or MeOH or mixtures with water in alkaline conditions was not developed and exploited further to obtain size-controlled Au NPs.

4. Materials and Methods

4.1. Chemicals

All chemicals were used as received: HAuCl4·3H2O (Sigma Aldrich); NaOH (Puriss., Sigma-Aldrich); water (Milli-Q, Millipore, resistivity of >18.2 MΩ·cm, total organic carbon (TOC) <5 ppb); methanol (MeOH, 99.8%, VWR); ethanol (EtOH, absolute, VWR); ethylene glycol (EG, 99+%, Sigma-Aldrich); glycerol (bi-distilled, 99.5% VWR); trisodium citrate dihydrate (Na3Ct, 99%, Alfa Aesar); lithium citrate tribasic tetrahydrate (Li3Ct, 99.5%, Sigma Aldrich); polyvinylpyrrolidone (PVP, Alfa Aesar, MW: 58,000).

4.2. Surfactant-Free Au NP Synthesis

The general recipe reported in [14] was followed. The stock solution concentration of HAuCl4 in water was 20 mM, the stock in solution of NaOH in water was at 57 mM. The final concentrations for the syntheses were 0.5 mM HAuCl4, 2 mM NaOH (so NaOH/Au molar ratio of 4) in typically 30 v.% ROH (or else as indicated) in water for a total volume of 13 mL (before volume contraction). All final volumes, v.% and concentrations are expressed before taking into account volume contraction, for instance observed upon mixing EtOH and water. In all cases, the HAuCl4 solution was added last. The syntheses were performed for 24 h at RT with stirring in PP centrifuges tubes at ambient light and pressure before the dispersions were stored in a fridge at ca. 4 °C. The magnetic stirring bars were cleaned with aqua regia between experiments [4].

4.3. Au NP Synthesis in Presence of Additives

Following up on the study performed in [14], we also investigated the effect of the age of the stock solution in the case where additives were used in the synthesis. The syntheses were performed, as described in the previous paragraph, but 2.5 mM of additive (PVP, Li3Ct or Na3Ct, as indicated) was also added from an aqueous stock solution. See also more information in Supplementary Materials.

4.4. Characterization

4.4.1. Transmission Electron Microscopy (TEM)

The as prepared Au NP dispersions, or diluted in EtOH, were dropped on nickel or copper TEM grids (Quantifoil) and left to dry at RT before imaging on a Jeol 2100 operated at 200 kV. The samples were characterized by recording micrographs in at least three randomly selected areas of the grid and at least three different magnifications. The size of the NPs was evaluated using the ImageJ software by measuring at least 30 NPs for the largest NPs (>50 nm), and more typically 100–200 NPs (for smaller NPs). The values reported below are number weighted sizes (Feret mean diameters) defined as: d N = ∑ d i N , and σ as the standard deviation related to dN. In the Supplementary Materials, the Sauter mean diameter and De Brouckere mean diameters as well as the polydispersity index are defined, and their values also reported. Upon TEM analysis, different types of structures were observed. If no indication is given, it means that the NPs are spherical. A note ‘Network’, ‘Worm’, ‘Chunks’ means that the structures look as per the classification proposed in [14] also illustrated above.

4.4.2. UV-Vis Spectrometry

UV–vis spectra were acquired with a Lambda 1050 UV/VIS/NIR absorption spectrometer (PerkinElmer). The as-prepared solutions were placed in quartz UV-vis cuvettes with a 1 cm path length and spectra recorded in the 200–800 nm range at ca. 1 nm s−1. As baseline, a measurement for a solvent mixture with the same water:ROH ratio as the sample was used. Au NMs present a rich UV-vis spectrum. Several metrics used in the literature that capture different features of the UV-vis data are reported in the tables below and in Supplementary Materials, see also [14].
λspr is the wavelength at which a spr signal is observed characterized by a corresponding local maximum in absorption intensity (Aspr). λspr values gives an indication of the size of spherical NPs. In a first approximation, for 525 < λspr < 579 nm, lower λspr correspond to smaller NPs [30]. The broadness of the spr peak is given by the relative width at 90% of Aspr (Δλ/λspr at 90% of Aspr) [31]. Aspr/A450, the ratio between absorption the intensity at the spr and the absorption intensity at 450 nm, gives an indication of the size of the NMs: a lower value corresponds to smaller NPs [30]. The relative intensity measured at 400 nm (A400) was suggested to indicate the relative amount of Au0 in the sample and so provides an estimation of the yield of the synthesis [25]. A650/Aspr, the ratio between the absorption intensity at 650 nm and the absorption the intensity at the spr, indicates the extend of aggregation of the NPs [32,33]. The higher this ratio, the more aggregated the NPs—it must be noted that this is technically relevant only if the NPs are characterized by a well-defined plasmon resonance peak. A380/A800, the ratio between the absorption intensity at 380 nm and the absorption the intensity at 800 nm, indicates the stability of the colloids and was used to compare different samples. The most stable colloids display a higher ratio [34].

5. Conclusions

In light of the documented effects of using an old stock solution of base, we encourage the use of relatively freshly prepared base solutions and recommend storage of the base stock solutions in plastic-ware, such as PP centrifuge tubes, to improve reproducibility towards fine size and shape control for the detailed RT synthesis of surfactant-free Au NPs using ROH as reducing agents. Since cations have been shown to play a role in the synthesis and stability of surfactant-free colloidal NPs [35], it is an open question if this effect related to ageing is screened of amplified using different bases such as LiOH or KOH. In all cases, the results illustrate the need for carefully controlling all steps of the synthesis of Au NPs and the results call for more detailed protocols and studies on the influence of cleaning procedures and/or reagents grade.

6. Patents

The presented nanotechnology is subject to a patent application. Applicant: University of Copenhagen; Inventors: Jonathan Quinson, Kirsten M. Ø. Jensen; Application number: EP21193770; Status: Patent filed.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11040140/s1, Details on TEM and UV-vis measurements and analysis; Table S1: Effect of using fresh or aged stock solutions of base for the synthesis of surfactant-free Au NPs, for different ROH and different v.% of ROH; Table S2: Effect of using fresh or aged stock solutions of base for the synthesis of surfactant-free Au NPs, for different alcohols; Table S3: Effect of using fresh or aged stock solutions of base for the synthesis of Au NPs using additives; Figure S1: UV-vis characterization of Au NMs obtained by surfactant-free RT synthesis using 30 v.% ethylene glycol as reducing agent prepared using a fresh or an old stock solution of NaOH. Reference [14] is cited in the supplementary materials.

Funding

This research was funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 840523 (CoSolCat). The APC was alleviated.

Data Availability Statement

All data are available upon request and/or available in the Supplementary Materials.

Acknowledgments

L. Theil Kuhn and S. B. Simonsen, Technical University of Denmark (DTU), Denmark, are thanked for access to TEM facilities.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Inorganics 11 00140 g001 550
Figure 1. Schematic representation of the surfactant-free RT synthesis investigated here and overview of the present study.
Figure 1. Schematic representation of the surfactant-free RT synthesis investigated here and overview of the present study.
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Figure 2. Characterization of Au NMs obtained by a surfactant-free RT synthesis using 30 v.% (a–c) EtOH and (d–f) MeOH as reducing agent. (a–e) TEM of samples prepared using (a,d) a fresh or (b,e) an old stock solution of NaOH and (c,f) UV-vis characterization of the samples, as indicated.
Figure 2. Characterization of Au NMs obtained by a surfactant-free RT synthesis using 30 v.% (a–c) EtOH and (d–f) MeOH as reducing agent. (a–e) TEM of samples prepared using (a,d) a fresh or (b,e) an old stock solution of NaOH and (c,f) UV-vis characterization of the samples, as indicated.
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Figure 3. UV-vis characterization of Au NMs obtained by a RT synthesis using different surfactants as indicated and 30 v.% (a–c) EtOH and (d–f) MeOH as the reducing agent and prepared using a fresh or an old stock solution of NaOH and additives (Li3Ct, Na3Ct, PVP), as indicated.
Figure 3. UV-vis characterization of Au NMs obtained by a RT synthesis using different surfactants as indicated and 30 v.% (a–c) EtOH and (d–f) MeOH as the reducing agent and prepared using a fresh or an old stock solution of NaOH and additives (Li3Ct, Na3Ct, PVP), as indicated.
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Figure 4. Characterization of Au NMs obtained a surfactant-free RT synthesis using 30 v.% (a–c) glycerol as reducing agent. (a,b) TEM micrographs of samples prepared using (a) a fresh or (b) an old stock solution of NaOH and (c) UV-vis characterization of the samples, as indicated.
Figure 4. Characterization of Au NMs obtained a surfactant-free RT synthesis using 30 v.% (a–c) glycerol as reducing agent. (a,b) TEM micrographs of samples prepared using (a) a fresh or (b) an old stock solution of NaOH and (c) UV-vis characterization of the samples, as indicated.
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Table
Table 1. Effect of using a fresh or and old stock solution of NaOH for the synthesis of RT surfactant-free Au NPs, for different ROHs and different v.% of ROH. UV-vis and TEM characterization for syntheses performed with 2 mM NaOH and 0.5 mM HAuCl4 (NaOH/Au molar ratio of 4) for a total volume of 13 mL and 24 h at RT. A more complete table is provided in Table S1.
Table 1. Effect of using a fresh or and old stock solution of NaOH for the synthesis of RT surfactant-free Au NPs, for different ROHs and different v.% of ROH. UV-vis and TEM characterization for syntheses performed with 2 mM NaOH and 0.5 mM HAuCl4 (NaOH/Au molar ratio of 4) for a total volume of 13 mL and 24 h at RT. A more complete table is provided in Table S1.
ROHROH
v.%		NaOHλspr/nm
(Δλ/λspr)		Relative
Yield *		A650/AsprdN/nmData
from
EtOH10Old612 (x.x%)0.260.97XThis work
Fresh530 (7.0%)1.000.209.1 ± 6.4[14]
30Old544 (15.6%)0.750.78Network (>20)This work
Fresh517 (6.4%)0.640.129.9 ± 2.3[14]
MeOH10Old584 (x.x%)0.15XXThis work
Fresh570 (x.x%)0.650.96Network (15)[14]
30Old557 (15.3%)0.810.77Network (10)This work
Fresh531 (6.3%)1.000.1721.2 ± 7.1[14]
50OldXXXNetwork (15)This work
Fresh542 (8.4%)0.740.4728.3 ± 11.1[14]
* evaluated as the ratio of A400 for the sample and the maximum values of A400 for the dataset for a given ROH. The number in parenthesis after ‘Network’ gives an indication of the size in nm. A ‘X’ means that the values could not be measured (too broad peak and/or too low signal intensity).
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Quinson, J. On the Importance of Fresh Stock Solutions for Surfactant-Free Colloidal Syntheses of Gold Nanoparticles in Alkaline Alcohol and Water Mixtures. Inorganics 2023, 11, 140. https://doi.org/10.3390/inorganics11040140

AMA Style

Quinson J. On the Importance of Fresh Stock Solutions for Surfactant-Free Colloidal Syntheses of Gold Nanoparticles in Alkaline Alcohol and Water Mixtures. Inorganics. 2023; 11(4):140. https://doi.org/10.3390/inorganics11040140

Chicago/Turabian Style

Quinson, Jonathan. 2023. "On the Importance of Fresh Stock Solutions for Surfactant-Free Colloidal Syntheses of Gold Nanoparticles in Alkaline Alcohol and Water Mixtures" Inorganics 11, no. 4: 140. https://doi.org/10.3390/inorganics11040140

APA Style

Quinson, J. (2023). On the Importance of Fresh Stock Solutions for Surfactant-Free Colloidal Syntheses of Gold Nanoparticles in Alkaline Alcohol and Water Mixtures. Inorganics, 11(4), 140. https://doi.org/10.3390/inorganics11040140

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