Stober Method for Silica Nanoparticles

Research will be limited to:
- Material: Silica Nanoparticles
- Method: Stober Method
- Chemicals: TEOS, Ammonia, Water, Ethanol
- Particle Size: 50-500 nm range

Strategy: I will investigate the quantitative relationships between the concentrations of TEOS, ammonia, and water in ethanol to establish a predictive model for silica nanoparticle size within the 50-500 nm range. The focus will be on identifying specific molar ratios and experimental protocols that allow for precise tuning, as well as reviewing established nomograms or empirical equations (like the Bogush et al. model) that correlate these parameters.
Queries:
- Stober method silica nanoparticle size tuning TEOS ammonia water ethanol 50-500nm
- Bogush et al silica nanoparticle size correlation TEOS ammonia concentration
- Stober process molar ratio water ammonia ethanol particle size distribution
- optimization of Stober method for monodisperse silica nanoparticles 50nm to 500nm
- influence of ammonia and water concentration on silica nanoparticle diameter Stober method

Executing 5 concurrent searches:
- Stober method silica nanoparticle size tuning TEOS ammonia water ethanol 50-500nm
- Bogush et al silica nanoparticle size correlation TEOS ammonia concentration
- Stober process molar ratio water ammonia ethanol particle size distribution
- optimization of Stober method for monodisperse silica nanoparticles 50nm to 500nm
- influence of ammonia and water concentration on silica nanoparticle diameter Stober method

Found 5 papers. Key excerpts:

Synthesis of Stober silica nanoparticles in solvent environments with different Hansen solubility parameters — Pavlo Sivolapov et al., 2022

Recently, technologies that use silica nanoparticles have become very popular. Compared with other materials, such particles possess a number of significant advantages: uniformity of size, composition and shape [1] , a surface favorable to modifications, and absence of harmful effects on the human body [2] . The most widespread and frequently used technique for obtaining the silica particles of controlled size is the synthesis proposed by a group of scientists led by V. Stober in 1968 [3] . This technique includes the hydrolysis of tetraethoxysilane (TEOS) in alcohol-water solution, followed by condensation of the resulting silanols. Usually, ammonia is used to adjust the pH of the medium (which should be equal to 11–13 [3] ) in the reaction mixture, which also performs a catalytic function [4] . However, sometimes this synthesis is also carried out in an acidic medium at a pH of 1–4 by adding HCl and NaF. It is possible to control the size and shape of the silicon dioxide particles through the variation of synthesis ratios and conditions and the possibility to obtain porous particles [5] . Typically, the size of silica particles obtained in the Stober synthesis is falling into the range from 20 to 800 nm [6] , [7] , [8] .
The described method of synthesis is mentioned in this paper only to demonstrate the possibility of obtaining SiO 2 particles in non-trivial conditions. Neither the substances nor the approach proposed in [5] were used in this work. A feature of this method is the ability to separate the stages of hydrolysis and condensation of silica particles, which allows more accurate adjustment of both the size of the particles and their morphology, including pore size. It should be noted that this effect does not occur with increasing of pH level during hydrolysis because NaF acts as a condensation catalyst and is added after the hydrolysis reaction is over. The probable dissolving of SiO 2 particles by formed during synthesis hydrofluoric acid is also not mentioned in the above work [5] . On the contrary, the authors note that the yield of synthesis is almost 100%. This may be due to the peculiarities of the interaction of TEOS with surfactants, which are added to the reaction mixture in the first stage of synthesis. This interaction is discussed in detail in another paper written by the same team of researchers [9]

, based on which we can assume that HF ​​does not interact directly with SiO 2 , or if such a reaction has place, then vey slightly.
Such particles are often used in drug delivery due to their well-developed surface and the possibility of their modification by attaching various functional groups [10] , [11] , [12] . In addition, silicon dioxide nanoparticles are widely used as biosensors [13] , catalysts [14] , and aerogels [15] , [16] .
The next promising area of application of nanosized silica is the obtaining of hierarchical superhydrophobic surfaces [17] , [18] . Hierarchical coatings are those consisting of two layers formed by micro- and nanoscale particles. The microlayer is usually formed by mineral fillers, while the nanolayer is formed by hydrophobized silica particles. This structure significantly increases the water-repellent properties of coatings, while imparting mechanical strength to them [19] , [20] .
However, the majority of stated applications requires the determined particle size of material, with the narrow range distribution. This fact causes the demand of synthetic instruments to achieve the target particle size distribution. As it was shown previously [21] , existing techniques include the variation of the temperature, TEOS, ammonia and water content, as well as the reaction time. There was also described an approach of the reaction media dielectric constants variation using different water miscible alcohols, that resulted in the fact that the particle size decreases with the decrease of the media polarity [22] , [23] .
The present study is aimed to the broadening of the description of the influence of synthetic environment on the produced silica particles properties in the scope of conventional solvent thermodynamic characterization approaches. The novelty of the research lies in the utilization of Hansen solubility theory [24] , that considers separately the dispersion, polar and hydrogen intermolecular interactions and provide additional dimensions for the variation of Stober synthesis conditions.
The objectives of this work include the implementation of Stober syntheses series in media with different thermodynamic parameters, determination of the product chemical composition and purity, characterization of the size distribution of obtained nanoparticles, obtaining the respective particle size curves and the determination of the influence of separate Hansen solubility parameters.
High-purity Dynasylan A SQ (Evonik, Germany)

TEOS (>99.9%) was used as the main component of Stober's synthesis. The following solvents were also used: isopropanol, isobutanol, tetrahydrofuran, ethyl acetate, butyl acetate and xylene with a purity of at least 99%. All solvents were purchased from VWR International (USA). The catalyst was ammonia in the form of an aqueous solution with a concentration of 28–30.0% (Sigma Aldrich, USA). Deionized water was used to carry out the hydrolysis reaction.
The literature indicates a lot of variations in the Stober synthesis [25] , [26] , so the authors of this study decided to take the formulation described in [27] as the initial one. Accordingly, for the synthesis of silicon dioxide nanoparticles, 1.5 ml of TEOS, 1.7 ml of ammonia, 1 ml of deionized water, and 50 ml of a solvent (mixture of alcohol and solvent) were added to the reaction medium. The listed components were mixed on a magnetic stirrer for 3 h at a temperature of 40 °C, after which an additional 1 ml of TEOS was added, and the reaction was continued under the same conditions for another 2 h.
An additional portion of TEOS was added to increase the yield of the reaction. According to studies [28] , the number of formed SiO 2 particles with a single addition of TEOS, regardless of its amount will be less than if re-loading this substance into the reaction mixture had place. It has also been observed that this procedure results in the formation of larger nanoparticles, but their morphology and size distribution remain the same. The nature of this phenomenon may be related to the aggregation mechanism of the formation of silica particles by recovering hydrolysis and condensation reactions. Thus, with a constant number of nuclei, an increase in the amount of TEOS will lead to an enlarging of existing particles, which practically will be characterized by a higher mass of the reaction product.
After the reaction, the system was washed with deionized water several (up to 10) times, after which the remaining solvents were removed using a rotary evaporator.
To obtain nanoparticles of different sizes, 4 pairs of solvents were used, and one of the solvents in each

Growth of SiO2 microparticles by using modified Stὂber method: Effect of ammonia solution concentration and TEOS concentration — Shrestha Bhattacharya et al., 2019

mobilized a worldwide interest in the last few decades. In this report a classical method known as the Stὂber method has been used to synthesize silica microspheres. These microparticles have been synthesized by the reaction of tetraethyl orthosilicate (Si(OC2H5)4, TEOS) (silica precursor) with water in an alcoholic medium (e.g. ethanol) in the presence of KCl electrolyte and ammonia as a catalyst. It has been observed that the size of the microparticles closely depends on the amount of the TEOS and ammonia. A decrease in the size of micro particles from 2.1µm to 1.7µm has been confirmed as the amount of TEOS increases from 3.5 ml to 6.4 ml respectively. In similar way a decrease in the diameter of the micro particles from 2.1 µm to 1.7 µm has been observed with increase in the ammonia content from 3 ml to 9 ml.
INTRODUCTION
The unique structural features of the silica micro particles have attracted a large attention since the last few decades. It is now being used for a large number of applications which include lithium ion batteries, catalysis, drug delivery, anti-reflective coating materials and also for cosmetics [1-5]. Due to mobility and high mechanical strength [6] SiO2 particles are also used for column packing, structural ceramics material ink additives, etc. Beside these applications SiO2 micro particles are also used to fabricate microwires solar cell using nanosphere lithography technique. In order to get different sizes of the silica particles, various methods of preparations have been adopted. Among the various methods used to prepare silica microspheres some are mechanical alloying method, micro emulsion method, hydrothermal synthesis method, precipitation, sol gel method and radiation synthesis method etc. [7-9]. The physical as well as the optical properties of the silica particles depend on the size of the particles.
Mostly the solution gelation commonly known as sol gel method is preferred over all other methods as this method involves simple chemistry and involves low cost techniques. In the sol-gel method a classical method known as the Stὂber method is used to produce silica spheres [10, 11].
In this process silica precursor tetraethyl orthosilicate (Si(OC2H5

)4, TEOS) is first reacted with water in an alcoholic medium typically ethanol in the presence of ammonia as a catalyst [12, 13] and also KCl electrolyte. It has been found that adding KCl could effectively increase the size of the silica particles.
Recently Lei et al. [14] reported that using Stὂber method silica micro particles having a diameter of about 1-3 µm could be obtained. He tried two different experiments. In one the two solutions were allowed to react for 15 hours and after centrifugation and washing with ethanol he found that the diameter of the micro particles was about 1µm. Further they continued the reaction in which a solution containing ethanol and TEOS was injected for 6 hours
at a rate 0.1-0.2ml/min in a solution containing KCl, ethanol, water and ammonia and further the reaction was left for 5 hours. In this they synthesized micro particles of 2-3µm size.
In this work synthesis of SiO2 particles up to 2.1 m using the Stὂber process has been reported. Variation of the
size of the microparticles with the variation in the amounts of TEOS and ammonia has been investigated.
EXPERIMENTAL

Materials
TEOS was purchased from Sigma Aldrich. KCl and the ethanol were purchased from the Merck life Science Private Limited. The ammonia solution (28-30%) was also purchased from Merck life Science Private Limited. All the other chemicals and the reagents were used as they were received without any sort of purification.
Preparation of the SiO2 particles
SiO2 micro particles were synthesized using simple modified Stὂber method in an ethanol solution along with ammonia solution which is used as a catalyst. The reaction is carried out at room temperature in a 250 ml beaker with mechanical stirring of about 200 rpm. Two solutions were made. Solution A was a mixture of KCl (15mg), ethanol (60ml), Water (7ml) and ammonia solution. Ammonia solution was varied amount wise as 1.5 ml, 3 ml, 6 ml and 9 ml respectively. Solution B was a mixture of TEOS (3.5ml, 6.4ml) and Ethanol (38.5ml, 38.6ml). The solution B was supplied to solution A using a syringe pump. After

the injection and further reaction, the micro particles were collected in a centrifuge tube and purified by centrifugation and washing by ethanol three times. Finally, the micro particles were dried either by natural evaporation or by evaporating the ethanol using hotplate at about 80 0 C. The micro particles obtained in this way are then dried and dispersed in DMF. They are then spin coated on the cleaned Si substrates.
Characterization
The size and the distribution of the silica micro particles synthesized are then viewed by an optical microscope. However high resolution images of the microspheres were taken using scanning electron microscope (SEM, model LEO 440 VP).
RESULTS AND DISCUSSION
The silica micro particles obtained from the experiment by different variations were analyzed using SEM
Effect of TEOS concentration on the Silica Particle Size
The amount of TEOS in a specific reaction also plays an effective role in controlling the diameter of the silica
particles. The concentration of TEOS is varied from 3.5 ml to 6.4 ml.
It could be noticed from Figure 1 that there is a slight change in the size of micro particles has occurred from 2.1µm to 1.7µm as the amount of TEOS increases from 3.5ml to 6.4ml respectively by keeping all other reactants amount same. This is mainly because the initial concentration of TEOS is inversely proportional to the size of micro particles. That means higher the concentration there is smaller particles due to greater number of nucleation sites but with a greater spread of sizes.
Effect of Ammonia concentration on Silica Particle Size
Figure 2 shows that with the increase in ammonia content from 3 ml to 9 ml in the solution A there is a decrease in the diameter of the micro particles from 2.1 µm to 1.7 µm. In this experiment the ammonia is mainly used as a catalyst. With the increase in ammonia concentration the quantity of the silica nuclei in the initial nucleation increases, resulting in decrease in particle size.
CONCLUSION
Silica microspheres are synthesized using modified Stὂber method. The diameter of the silica micro particles that are obtained by supplying TEOS continuously using syringe pump is much larger than traditional method. It has also been seen that the supply rate of TEOS also plays an important role in the particle size. The amount of TEOS concentration as

Controllable synthesis of SiO2 nanoparticles: effects of ammonia and tetraethyl orthosilicate concentration — Yue Yan et al., 2016

1 Introduction

     SiO2 nanoparticles play important roles in several applications, including catalysis [[1], [2]], lubrication [[3]], ceramics [[4]], and photographic emulsions [[5]], because of their excellent physical and chemical properties. Moreover, further functionalisation with numerous agents can improve these particles for various applications [[6]], which is an advantage of nanomaterials. Among many remarkable properties, the high biocompatibility of SiO2 nanoparticles lends itself to potential applications in drug delivery systems. In this strategy of using SiO2 nanoparticles as a drug carrier, an increasing amount of evidence indicates that the size of the SiO2 nanoparticles plays a vital role in their distribution in the lymphatic system and for targeting osmosis [[7], [8]]. Because of the known ‘enhanced permeability and retention effect [[9]]’ in tumours, a tumour's vasculature is selective toward molecules of different sizes. Recent data have shown that in the correct size range, SiO2 nanoparticles will be prevented from exiting normal vasculature but can be dosed into tumour regions [[10]]. Ghandehari showed that 20–60 mm SiO2 particles exhibit good expression, e.g. biodistribution, cell uptake and tissue penetration in vivo [[11]]. Thus, controlling the size of SiO2 nanoparticles is a pivotal issue for the use of SiO2 particles in drug delivery.

     Since Kolbe [[12]] discovered the formation of monodisperse SiO2 particles through the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol with ammonia as a catalyst, this reaction system has been investigated in many studies [[13]-[17]]. Bogush et al. [[13]] postulated that the growth of silica particles mainly depends on the aggregation of small silica seeds. Van Blaaderen et al. [[18]] reported that surface-reaction-limited condensation controls the particle formation. The most common method is the so-called Stöber [[19]] method, which describes hydrolysis and condensation as follows:











     Accordingly, the size and size distribution of SiO2 nanoparticles depend on the synthetic conditions, including the amounts of water, ammonia hydroxide, TEOS and different alcohol types. However, principles for guiding the synthesis have been limited thus far, and there is no model for controlling the particle size. Thus, determining the rules for tuning the size of SiO2 nanoparticles by changing the synthetic conditions has been a challenge.

     In this article, the effects of ammonia concentration, TEOS concentration, water loading and alcohol type on the size and dispersion of synthesised SiO2 nanoparticles were investigated. Linear relationships were found between the SiO2 particle size and the concentrations of ammonia and TEOS, which can be used to control the size of SiO2 nanoparticles during synthesis.

2 Experimental

        2.1 Materials

        The following compounds were of analytical grade and used as received without further purification: ammonium hydroxide (Changzheng Pharm, 25–28%), tetraethoxysilane (Aldrich, ≥99.0%), methanol (Sinopharm, ≥99.5%), ethanol (Sinopharm, ≥99.5%), isopropanol (Sinopharm, ≥99.5%), and n-butanol (Sinopharm, ≥99.5%). Deionised water was prepared in house.






        2.2 Synthesis of SiO2 nanoparticles

        SiO2 particles were prepared according to the Stöber method. Accurately measured TEOS and alcohol were initially mixed in a round-bottom flask and stirred for 10 min at room temperature with a rotation speed of 1000 rpm. A certain amount of deionised water and ammonium hydroxide were added immediately under stirring. After 12 h, the SiO2 particles were separated by centrifugation and dried.




        2.3 Characterisation of SiO2 nanoparticles

        The size and dispersion of the SiO2 particles were measured using a Malvern Zetasizer Nano ZS and a Hitachi S4800 high-resolution scanning electron microscope (SEM). To prepare the sample for dynamic light scattering, 0.05 g of SiO2 powder was dispersed in 10 ml of ethanol and sonicated for 10 min before being measured by the Malvern Zetasizer Nano ZS. The samples were prepared for SEM by sputtering the SiO2 powder with gold and then placing the powder on aluminium foil. The SEM images were obtained with a magnifying power of 100 k.

3 Results and discussion

        3.1 Effect of ammonia concentration on the size and dispersion of SiO2 nanoparticles

        Fig. 1 shows the size and dispersion of SiO2 nanoparticles prepared using

Controlling particle size in the Stöber process and incorporation of calcium — Sarah L. Greasley et al., 2016

Introduction

Bioactive glass nanoparticles have great potential for delivery of therapeutic cations and they can be made by sol-gel . Bioactive glasses can act as delivery vehicles for sustained delivery of active ions that have therapeutic benefit . The benefit of biodegradable glasses over polymers is that the ions are incorporated into the glass composition, due to the amorphous structure, and they are released at a sustained rate as the glass dissolves . Nanoparticles have particular benefit for intracellular delivery of ions . Sol-gel processing methods are used to produce silica networks by hydrolysis and condensation reactions . A benefit over meltquench glasses is that there is potentially more control over composition at lower processing temperatures. A silicate precursor is required, which typically takes the form of a silicon alkoxide, such as tetraethyl orthosilicate (TEOS). Silicon alkoxides hydrolyse under both acidic and basic conditions, after which polycondenation occurs and Si-O-Si bonds start to form, creating a sol of dispersed nanoparticles. In the acid-catalysed system, particles aggregate as condensation continues to form a three-dimensional gel network [4,5]. However, under basic conditions, the presence of OH -ions results in repulsive forces making it possible to synthesise monodispersed spherical nanoparticles . Stober pioneered this system, producing monodisperse silica spheres in the micron size range (from 0.05 -2 um) .
The Stober process is simple: Tetraethyl orthosilicate (TEOS) is added to a solution of water, alcohol and ammonium hydroxide under agitation. One of the advantages of the method is the ability to control particle size, distribution and morphology by systematic variation of reaction parameters . Other papers have been published on the synthesis of silica micro-and nanoparticles and have adapted the Stober process with various concentrations in their own investigations . However, the synthesis methods have become increasingly complex with no clear benefits. The process's high sensitivity to the effects of temperature, pH and reactant concentrations have affected reproducibility and consistent trends have not always been observed [10,14]. Whilst there have been attempts to demonstrate how each of these variables affects the final particle properties, most papers use different concentrations and processing methods, making comparison difficult. There is still much disagreement in the field and the original paper published by Stober et al. and that of

Bogush et al. remain the most comprehensive studies. The aim here is to incorporate cations in the Stober process while maintaining monodispersity in nanoparticles. In order to do that, it was first necessary to gain a comprehensive understanding of the effects of variables in the Stober process.
It is well known that the sol chemistry affects the rates of hydrolysis and condensation, which in turn affects nucleation, aggregation and growth of particles [4,12]. It is proposed that particle growth follows a nucleation and aggregation mechanism [4,17], in which initial negatively charged particles (<10 nm) are unstable due to their size, resulting in aggregation and a collective reduction in surface area. Competition between nucleation and aggregation is dependent on reactant concentrations and has a large effect on final particle properties such as size . Uniformity in particle size can be achieved with this mechanism as a result of size dependent aggregation rates [8,18] determined by colloid interaction potentials. The aggregation rate is fastest between small-large particles and slowest between large-large particles . Initial particles restructure through Ostwald ripening, in which aggregates dissolve and then reprecipitate, consuming smaller particles to form larger more stable ones. However, Ostwald ripening cannot account for the rapid growth of particles in the 45 -250 nm range. The most likely mechanism for the growth of larger particles is by reaction-limited monomer-cluster growth (RLMCA) , where the probability of monomer attachment is governed by local structure. There are many collisions between monomer (a silicate tetrahedron in this case) and cluster before a bond is formed, with all potential growth sites being sampled by the monomer. In basic conditions, all cluster sites are reactive and occupied with equal probability, therefore giving rise to spherical particles . It is predicted that RLMCA occurs on two length scales with the smaller particles now acting as monomers . This mechanism is supported by Feeney et al. , Lee et al. and Harris et al. . Once the soluble silica concentration has dropped below the critical nucleation concentration, it is hypothesised that monomer addition subsequently occurs on the surface of the aggregated particles in accordance with LaMer and Dinegar's theory of monomer addition , leading to smoothing of the colloid surface [8,12,16].
The incorporation of calcium into the nanoparticles is desirable in order to create

a bioactive system. Soluble silica and calcium ion release stimulates cells at a genetic level causing osteogenic cells to produce bone matrix [2,20,21]. The addition of calcium also increases glass dissolution by acting as a network modifier. Bioactive glass nanoparticles can also be incorporated into nanocomposites . Whilst the synthesis of monodispersed Stober silica nanoparticles has been achieved previously in literature, the addition of calcium significantly complicates the procedure and can cause the particles to become irregular in morphology or to agglomerate . While nominal compositions of particles are quoted in the literature, the final compositions are rarely ratified by analytical techniques or only non-quantitative energy-dispersive X-ray spectroscopy (EDX) data is provided [3,25,27].
One of the reasons for the challenges in incorporating calcium into the silicate network is that it requires elevated temperature when calcium salts are used as the calcium precursor. Several groups have attempted to incorporate calcium into silica nanoparticles by using a two-step sol-gel process in which precursors of silica and calcium (TEOS and calcium nitrate respectively) are hydrolysed in an acidic solution before gelation under alkaline conditions . While EDX data showed calcium was present in the particles and they induced HCA formation in simulated body fluid (SBF), little or no quantitative analysis was performed on the final elemental composition. The particles are also seen to be aggregated and irregular in both size and shape [24,25,27]. Labbaf et al. incorporated calcium by adapting the process used by Zhao et al. in which Boltorn(tm) polymer was used as a template in the sol-gel process before being burnt out during calcination. The submicron particles were shown to be of composition 86 mol% SiO 2 and 14 mol% CaO, verified by quantitative inductively coupled plasma optical emission spectroscopy (ICP) analysis . However, despite being spherical and dispersed, the resulting particles did not show a homogeneous size distribution even after optimisation of the polymer:TEOS ratio. Some papers have attempted to improve the dispersion or shape of the nanoparticles by use of a surfactant [25,28]. However, this can inhibit calcium diffusion, therefore limiting the amount of calcium that enters the glass .
Calcium nitrate is usually used as the calcium source in the sol-gel synthesis of bioactive

“DIY” Silica Nanoparticles: Exploring the Scope of a Simplified Synthetic Procedure and Absorbance-Based Diameter Measurements — Łukasz Tabisz et al., 2020

1. IntroductionIn recent years, the chemistry behind the synthesis of new materials has come to rival the most long-standing and expansive fields of organic, inorganic, and analytical chemistry. As is true with most sciences, however, these disciplines no longer have well-defined boundaries. In particular, as we study ever smaller particles—venturing into the nanometer range—we can now directly inspect (and question) the blurred distinction between molecules and materials [1,2,3]. Of particular importance to the present paper is the observation that while cutting-edge research in the chemistry of materials continues, its previous accomplishments are of tremendous use to other branches of chemistry, e.g., chromatography, catalysis, and the study of interfacial phenomena [4,5,6]. However, more innovative ideas call for a solution that must be individually tailored and made, not simply bought—a task that many non-material chemists believe is beyond their reach. This notion should be challenged, and simplified methods of synthesis and analysis of (nano)materials need to be developed and disseminated, in parallel with sophisticated research requiring uncommon equipment.This “backtracking” can yield unexpected insights and solutions [7,8,9], of value even to those far more specialized in the field. The original Stöber process has recently seen its 50th anniversary and remains one of the cornerstones of sol-gel chemistry [10].Much research has drawn on the idea of the bottom-up synthesis of nanoparticles [11,12,13], and even the original method is regularly revisited, highlighting its accessibility and ease of modification [14,15,16]. It can still be used as a reliable source of silica particles, with a very narrow size distribution, in the range of 10–800 nanometers [16,17] (larger are also reported [18,19]). The appeal of a simple, one-pot preparation of such uniform (and such small) spheres is not limited to didactics. While the ease of preparation of Stöber silica cannot be disputed, a multitude of factors still influence the resulting size and homogeneity of material, as shown by a number of papers [14,16,17]. In practice, in large-scale experiments with many samples, it is quite common to encounter “botched” runs, and untrained hands, of course, exacerbate the risk. Therefore, during our work, we have

encountered the rising need for a streamlined synthesis and size-determination procedure, in order to quickly eliminate errors in nanoparticle preparation. As for synthesis the most important factor is the ammonia/water/tetraethylorthosilicate (TEOS) ratio, we decided to reduce the number of reagents and swap the ethanolic ammonia solution (commonly encountered in published procedures) for an aqueous one, which is perhaps one of the most rudimentary laboratory commodities, along with setting the TEOS concentration to a fixed value. More importantly, following the observation of a seemingly linear increase in the perceived turbidity of post-reaction mixtures, we aimed to establish if, and in what circumstances, simple UV-Vis spectrophotometer measurements could be used for the routine and straightforward determination of Stöber nanosilica (and, by analogy, possibly other nanoparticle) sizes. The theory of light scattering is well-established and for homogenous spheres in particular it has been described by Mie as a solution to Maxwell’s equations [20,21,22]. Many authors have tackled the problem of using turbidity measurements—in series, using approximations or different wavelengths [23,24,25]—for the determination of the average size of material particles, which is in itself a crucial and dynamic field of research [26,27]. A large body of work is also devoted to the limitations of that methodology when it comes to giving proper information about the size distribution in samples [28,29]. The idea of using a modified UV-Vis spectrophotometer, in combination with Mie theory-based computer calculations, is also not new [25,30,31]. To the best of our knowledge, however, bridging the gap between that rigorous and complicated model with feasible measurements using an instrument primarily designed for absorption determinations, while retaining the simplicity of that determination (using a convenient linear approximation), has not yet been reported. 2. Materials and Methods 2.1. ReagentsEthanol (EtOH, 96% and 99.8%) and ammonia solution (NH3, 25% in water) were obtained from Avantor Performance Materials POCH, Poland S.A., Gliwice, Poland. Tetraethylorthosilicate (TEOS) and hydrofluoric acid (HF, 48% in water) were purchased from Merck (Kenilworth, NJ, United States). All chemicals were used

without further purification but were either freshly opened (ammonia and ethanol) for each series of experiments or stored under argon (TEOS) in between them. 2.2. Equipment and SoftwareThe size distribution of particles was analyzed using transmission electron microscopy (JEM 1200-EX, JEOL Co., Tokyo, Japan). For each sample, at least four micrographs from different sections were taken, in at least two different magnifications, and analyzed with ImageJ (Fiji) free software (ImageJ 2.0.0-rc-69/1.52p, fiji.sc). The samples were randomly assigned a number (scrambled) in order to perform blinded data analysis. Theoretical calculations were run using Maple software (Maple 16, Maplesoft, a division of Waterloo Maple Inc., Waterloo, Ontario).Visual light absorption/transmission measurements were taken on three different spectrophotometers, as explained in detail in the following sections: Jenway 6305 UV-Vis spectrophotometer, Jenway 6400 Vis scanning spectrophotometer (Cole-Palmer Ltd., Vernon Hills, IL, USA), and Agilent 8453 Diode Array spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). 2.3. Preparation of Silica NanoparticlesA total of 44 nanoparticle samples were obtained in three separate experiments. The first set (18 samples) utilized standard 96% ethanol as the solvent, while the second and third used 99.8% ethanol (18 and 8 samples, respectively). The synthetic conditions were informed by previous reports [10,17,19], but specifically restricted due to our objectives. Only three reagents were used, with the volume of TEOS set to a constant 4% (v/v) and the amount of aqueous NH3 (25%) being the main variable (3–26% v/v); ethanol made up the rest of the reaction mixture. Silica syntheses were performed in 15 cm3 plastic screw-cap tubes, washed with 2% HF, and rinsed with deionized water.In a typical experiment, 9.6 cm3 of ethanol-ammonia solution (with varying proportions of NH3) was directly prepared in a tube equipped with a small magnetic stirring bar, closed, and placed in a small rack on top of a magnetic stirrer (set to 1000 rpm). After