Flow Cell Coupled Dynamic Light Scattering for Real-Time Monitoring of Nanoparticle Size During Liquid Phase Bottom-Up Synthesis

applied sciences

Communication Flow Cell Coupled Dynamic Light Scattering for Real-Time Monitoring of Nanoparticle Size during Liquid Phase Bottom-Up Synthesis

Nicole Meulendijks 1, Renz van Ee 1, Ralph Stevens 1, Maurice Mourad 1, Marcel Verheijen 2,3, Nils Kambly 4, Ricardo Armenta 4 and Pascal Buskens 1,5,6,* 1 The Netherlands Organisation for Applied Scientiﬁc Research (TNO), De Rondom 1, 5612 AP Eindhoven, the Netherlands; [email protected] (N.M.); [email protected] (R.v.E.); [email protected] (R.S.); [email protected] (M.M.) 2 Philips Innovation Labs, High Tech Campus 11, 5656 AE Eindhoven, the Netherlands; [email protected] or [email protected] 3 Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands 4 LS Instruments AG, Passage du Cardinal 1, CH-1700 Fribourg, Switzerland; [email protected] (N.K.); [email protected] (R.A.) 5 Hasselt University, Institute for Materials Research, Inorganic and Physical Chemistry, Agoralaan Building D, B-3590 Diepenbeek, Belgium 6 Zuyd University of Applied Sciences, Nieuw Eyckholt 300, Postbus 550, 6400 AN Heerlen, the Netherlands * Correspondence: [email protected] or [email protected]; Tel.: +31-888-662-990

Received: 18 December 2017; Accepted: 10 January 2018; Published: 13 January 2018 Featured Application: A unique set-up for real-time monitoring of the size of nanoparticles during bottom-up liquid phase synthesis is presented in this article. The analysis method applied to study the size of dispersed nanoparticles during synthesis is dynamic light scattering (DLS). In contrast to conventional DLS, the DLS set-up presented in this article comprises a modulated 3D cross correlation geometry, and therefore allows accurate measurements of particle size in flow at flow rates of at least up to 17 mL·min−1. This is essential for obtaining real-time information on the size of the dispersed nanoparticles. The DLS system could be connected to reactors of various sizes using the analysis loop presented in this article, which is coupled to a flow cell in the DLS set-up. Thus, the DLS set-up presented here is suited to study the nucleation and growth of nanoparticles in dispersion, facilitates a rational scale-up, and allows intervention in the production process of nanoparticle dispersions to minimize the number of off-spec batches.

Abstract: To tailor the properties of nanoparticles and nanocomposites, precise control over particle size is of vital importance. Real-time monitoring of particle size during bottom-up synthesis in liquids would allow a detailed study of particle nucleation and growth, which provides valuable insights in the mechanism of formation of the nanoparticles. Furthermore, it facilitates a rational scale-up, and would enable adequate intervention in the production process of nanoparticle dispersions to minimize the number of off-spec batches. Since real-time monitoring requires particle size measurements on dispersions in flow, conventional dynamic light scattering (DLS) techniques are not suited: they rely on single scattering and measure the Brownian motion of particles dispersed in a liquid. Here, we present a set-up that allows accurate measurements in real-time on flowing dispersions using a DLS technique based on modulated 3D cross-correlation. This technique uses two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume in order to extract only the single scattering information common to both. We connected the reactor to a flow-cell in the DLS equipment using a tailor-made analysis loop, and successfully demonstrated the complete set-up through monitoring of the size of spherical silica nanoparticles during Stöber synthesis in a water-alcohol mixture starting from the molecular precursor tetraethyl orthosilicate.

Appl. Sci. 2018, 8, 108; doi:10.3390/app8010108 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 108 2 of 10

Keywords: dynamic light scattering; 3D cross correlation; real-time analysis; colloids; nanoparticle synthesis

1. Introduction To tailor the properties of nanoparticles and composite materials comprising such nanoparticles, control of the particle size is of vital importance. Nanoparticles are used in products for a wide variety of applications ranging from performance polymers to pharmaceutics, food ingredients, cosmetics and coatings and paints [1–6]. Very often, such well-defined nanoparticles are produced in bottom-up liquid phase syntheses using e.g., redox or sol-gel type reactions. A detailed study of the nucleation and growth of such particles would provide valuable information on their mechanism of formation, and could furthermore be used for process optimization and scale up. For such a study, real-time measurements of the particle size are required. For production scale synthesis, such real-time measurements could also provide data that enable rational intervention in a production process, hence minimizing the number of off-spec batches and waste. Dynamic Light Scattering (DLS) is the standard technique for determining the size of nanoparticles in the range of a few nanometers to a few microns dispersed in liquids [7,8]. It makes use of a laser to study the time dependent scattering of small particles in liquid dispersions. Since such particles undergo Brownian motion, the distance between the scattering particles in the liquid constantly changes, which results in a fluctuating scattering intensity [9]. Analysis of these fluctuations yields information about the average size and size distribution of the nanoparticles. Since in conventional DLS experiments no external flow is imposed on the dispersion, the fluctuating scattering intensity solely correlates to the diffusion of particles, and does not comprise translational information. It can thus be used to determine the diffusion coefficients of the particles, and from that the particle size distribution through use of the Stokes-Einstein equation. Consequently, such conventional DLS techniques do not allow for real-time measurements on flowing or moving dispersions. DLS measurements rely on single scattering, which means that each photon should only be scattered once in the dispersion sample before it reaches the detector. Since this requires low particle concentrations, dispersion samples typically require dilution with additional solvent which is inconvenient and may even influence the state of the dispersed colloids. To overcome this, a significant amount of research was performed in the past decade focusing on suppressing multiple scattering. It showed that multiple scattering suppression in DLS measurements can e.g., be accomplished by using two lasers operating at different wavelengths, and two detectors having distinct bandpass filters to capture scattering information from each single laser. This technique is generally referred to as two-color DLS [10]. Another more simple and robust arrangement of the beam-detector pairs is possible by displacing them symmetrically in a third dimension above and below the scattering plane, which is known as the three-dimensional (3D) cross-correlation geometry [11]. Using this geometry, two scattering experiments are performed at the same time and on the same scattering volume using two beams and two detectors. The signals seen by the detectors are cross-correlated, and only photons that are scattered once will produce correlated intensity fluctuations on both detectors. In that way, the information on single scattered photons can be extracted, and used to determine that particle size distribution in turbid dispersions. However, an important drawback of this 3D technique is that each photon detector not only receives the desired information from its correspondingly positioned laser, but also undesired information from the second laser. Thus, cross-talk occurs between the two light scattering experiments. This drawback is overcome through temporal isolation of the two beam-detector pairs in modulated 3D cross-correlation DLS. Appl. Sci. 2018, 8, 108 3 of 10

Real-time DLS measurements moreover are challenging since a flow is imposed on the dispersion in laboratory and pilot-scale reactors, e.g., through stirring or pumping. Accurate particle sizing using DLS usually necessitates a resting fluid in the measured sample [12]. In actively mixed fluids, the diffusion is overlaid by a turbulent convection which prohibits diffusion measurements. Here, we demonstrate a set-up based on modulated 3D cross-correlation DLS, which is capable of separating the scattering information resulting from diffusional and translational motion of particles, and consequently allows flow cell coupled measurements on flowing nanoparticle dispersions [13]. In this setup, we pump a particle dispersion from the reactor through an analysis loop comprising a flow cell, and back into the reactor. We successfully validated the set-up presented in this article through monitoring of the size of spherical silica (SiO2) nanoparticles during Stöber synthesis starting from the molecular precursor tetraethyl orthosilicate (TEOS). Conventional off-line DLS on non-moving liquid dispersions using a state of art DLS instrument, and transmission electron microscopy (TEM) are used to validate the particle size information obtained with our newly developed flow cell coupled DLS instrument.

2. Materials and Methods

2.1. Materials Ethanol (EtOH) was obtained from Biosolve (Valkenswaard, the Netherlands) and used without further puriﬁcation. Tetraethyl orthosilicate (TEOS), potassium chloride (KCl) and ammonia (NH3, 30–33% in water) were obtained from Sigma Aldrich (Zwijndrecht, the Netherlands) and used without further puriﬁcation.

2.2. Synthesis As prototypical liquid phase bottom-up nanoparticle synthesis to demonstrate the capability of our real-time monitoring set-up, we selected the Stöber synthesis of spherical silica nanoparticles. This sol-gel synthesis is performed in water-alcohol mixtures. Under ambient conditions, the particles are formed through hydrolysis of TEOS and subsequent polycondensation over a period of several hours. Two sizes of silica nanoparticles were prepared following the procedures reported by Khan et al. [14] and Sato-Berru et al. [15]. For the preparation of silica particles with a radius of approximately 20 nm [9], referred to as “small particles”, EtOH (35 mL) and NH3 (3.0 mL) were mixed, after which a mixture of TEOS (6.0 mL) in EtOH (40 mL) was added under stirring. The experiment was carried out at room temperature under magnetic stirring at 500 rpm for 5 h. For the preparation of silica particles with a radius of approx. 150 nm [10], referred to as “large particles”, EtOH (45 mL), demineralized water (5.0 mL) and NH3 (3.0 mL) were mixed for 5 minutes, after which TEOS (1.5 mL) was added. This experiment was carried out at room temperature under magnetic stirring at 500 rpm for 3 h.

2.3. Analyses

2.3.1. Flow Cell Coupled DLS for Application in Real-Time Measurements of Nanoparticle Dispersions

For flow cell coupled real-time measurements of nanoparticle dispersions, we used a DLS set-up based on the 3D cross correlation geometry, as developed by LS Instruments. This set-up comprises a laser which emits light at a wavelength of 685 nm. This light is emitted and directed at the dispersion through a polarization-maintaining optical fiber. The light scattered by the nanoparticles at 90◦ with respect to the incident beam, and is collected by a second optical fiber. The latter is connected to a single-photon avalanche diode (SPAD) and the signal is analyzed by a LSI Correlator (LS Instruments). In combination with a flow cell, the DLS unit equipped with LsLab software version 1.0.4.0. was used for real-time particle size measurements. To perform measurements on moving liquid dispersions, a tailored analysis loop was prepared and applied to couple the DLS instrument to a stirred 2 L Appl. Sci. 2018, 8, 108 4 of 10 double-walled glass tank reactor. The DLS instrument was integrated in the analysis loop in direct connection with the stirred tank reactor, and adapted to accommodate a flow cell including tubing material to enable real-time measurements. A direct connection of the sample cell to the reactor via a tubing system, however, could easily transmit vibrations which would affect the particle motion in the liquid. For this reason, it was necessary to decouple and reduce the vibrations sufficiently, such that the Brownian motion of particles in the measured sample volume can be detected. Directly after the addition of precursor to the reaction mixture and start of the sol-gel reaction to form nanoparticles, the dispersion was pumped with a speed of 12.9 mL·min−1 through the DLS flow cell (type Starna, 71-F/Q/10) using a Watson pump with flexible tubing (type Inacom Tygon R3607, inner tubing diameter 2.79 mm), after which the measurement was started. The particle dispersion circulates through the flow cell back into the reaction mixture.

2.3.2. Off-Line Dynamic Light Scattering on Non-Moving Liquid Dispersion For off-line DLS measurements on non-moving liquid dispersions, we used a Malvern Zetasizer Nano ZS (ZEN 3600, λ = 633 nm, θ = 173◦, Malvern, United Kingdom) as a reference. For the measurement on non-moving dispersions, 2.0 mL was taken out of the reaction mixture and inserted in a polymer cuvette after which the cuvette is directly placed in the DLS instrument and the measurement is performed. The temperature was set to 25 ◦C, and the viscosity and refractive index values used were that of pure ethanol. For each measurement data were acquired during 30 s in three runs.

2.3.3. Transmission Electron Microscopy Particle size measurements of the newly developed instrument were compared to measurements obtained through transmission electron microscopy (TEM). The dimensions and composition of the particles were determined by TEM using a JEOL ARM200F (Tokyo, Japan) operated at 80 kV. To prepare samples for analysis, 4.0 µL of the particle dispersion in ethanol was placed on a copper grid coated with carbon. The grid was placed on a filter paper to absorb liquid flowing through the perforated film, and the sample was left to dry in air for a few minutes before being stored in a plastic container prior to the TEM analysis. All images were acquired in High Angle Annular Dark Field (HAADF)—Scanning TEM mode, allowing for a clear contrast between silica and the carbon support film because of the difference in atomic number. Simultaneous energy dispersive X-ray (EDX) analysis was used to confirm the constituents of the particles.

3. Results and Discussion The aim of our work was to design, build and validate a set-up for flow cell coupled DLS measurements that enables real-time monitoring of the size of nanoparticles during liquid phase synthesis. For flow cell coupled real-time measurements of dispersions, we used a DLS unit equipped with the 3D cross correlation geometry (the unit is based on the commercially available NanoLab 3D, LS Instruments AG, Fribourg, Switzerland). The technology applied in this instrument is based on two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume in order to extract only the single scattering information common to both (Figure1). It enables short measurement acquisition times (dark counts <250 s−1). To perform measurements in stirred fluids, a custom designed analysis loop was used to couple the DLS instrument to a stirred 2 L double-walled tank reactor. The dispersion is lead to the measurement module by means of an analysis loop (Figure2). Appl. Sci. 2018, 8, 108 5 of 10 Appl. Sci. 2018, 8, 108 5 of 10

Appl. Sci. 2018, 8, 108 5 of 10

Figure 1. Schematic representation of the dynamic light scattering (DLS) instrument equipped with Figure 1. Schematic representation of the dynamic light scattering (DLS) instrument equipped with 3D 3D cross correlation geometry. Reprinted with permission from LS Instruments AG. cross correlation geometry. Reprinted with permission from LS Instruments AG. Figure 1. Schematic representation of the dynamic light scattering (DLS) instrument equipped with 3D cross correlation geometry. Reprinted with permission from LS Instruments AG.

(a)

(b)

Figure 2. (a) Schematic representation of the analysis loop for coupling the DLS unit to the pilot reactor system and (b) detailed photograph of the(b setup.) Figure 2. (a) Schematic representation of the analysis loop for coupling the DLS unit to the pilot Figure 2. (a) Schematic representation of the analysis loop for coupling the DLS unit to the pilot reactor reactor system and (b) detailed photograph of the setup. system and (b) detailed photograph of the setup. Appl. Sci. 2018, 8, 108 6 of 10

In the analysis loop, the reaction mixture is circulated continuously using a Watson pump with ﬂexible tubing. The pump is placed after the cuvette to circumvent contamination of the sample with agglomerated particles due to scaling of the pump. After measurement, the DLS loop is rinsed by demineralized water to clean it before the next sample enters the measuring zone. Appl. Sci. 2018, 8, 108 6 of 10 To validate the set-up, we started monitoring the growth of silica nanoparticles in a Stöber synthesis startingIn the analysis from the loop, molecular the reaction precursor mixture is TEOScirculated in acontinuously mixture of using water, a Watson ethanol pump and with ammonia. This is aflexible prototypical tubing. The liquid pump phase is placed synthesis after the of cuvette nanoparticles. to circumvent In this contamination sol-gel synthesis, of the sample TEOS with partially hydrolyses,agglomerated and through particles subsequent due to scaling polycondensation of the pump. After silica measurement, nanoparticles the areDLS formed loop is rinsed as shown by in the reactiondemineralized Equations (1)–(3) water to [16 clean]. it before the next sample enters the measuring zone. To validate the set‐up, we started monitoring the growth of silica nanoparticles in a Stöber synthesis starting from the molecular precursor TEOS in a mixture of water, ethanol and ammonia. Si(OC H ) + xH O → Si(OC H ) (OH) + xC H OH (0 ≤ x ≤ 4) (1) This is a prototypical2 5 liquid4 phase2 synthesis of2 nanoparticles.5 4−x x In this2 sol5‐gel synthesis, TEOS partially hydrolyses, and through subsequent polycondensation silica nanoparticles are formed as shown in the reaction Equations (1)–(3)≡Si-O-H [16]. + H-O-Si ≡ → ≡ Si-O-Si ≡ +H2O (2)

Si(OC≡Si-OC2H5)42 +H x5H+2O H-O-Si  Si(OC≡2H →5)4 ≡−x(OH)Si-O-Six + xC≡2H+C5OH2H (05 ≤ OHx ≤ 4) (1) (3) This process proceeds according≡Si–O–H to + an H–O–Si aggregation-based ≡  ≡ Si–O–Si ≡ +H kinetic2O model [17,18]. The(2) size of the silica nanoparticles resulting≡Si–OC from2H this5 + H–O–Si synthesis≡  ≡ dependsSi–O–Si ≡ +C amongst2H5OH others on the concentration(3) of ammoniaThis and process TEOS. proceeds The according average to particle an aggregation size increases‐based kinetic with model increasing [17,18]. The concentration size of the of ammonia.silica The nanoparticles initial concentration resulting from ofthis TEOS synthesis is inversely depends amongst proportional others on to the concentration size of the resultingof silica nanoparticles.ammonia and TEOS. The average particle size increases with increasing concentration of ammonia. The Weinitial started concentration validating of the TEOS effect is inversely of flow proportional rate on the to particle the size of size the measured resulting silica in thenanoparticles. flow-cell coupled We started validating the effect of flow rate on the particle size measured in the flow‐cell coupled DLS set-up with 3D cross correlation geometry. Therefore, a 1 wt-% dispersion of “large” and “small” DLS set‐up with 3D cross correlation geometry. Therefore, a 1 wt‐% dispersion of “large” and “small” silica particlessilica particles of a radius of a radius of 157 of 157 nm nm and and 17.6 17.6 nm, nm, respectively,respectively, were were circulated circulated in the in analysis the analysis loop loop −1 comprisingcomprising the DLS the at DLS flow at flow rates ratesQ between Q between 0 and0 and 17 17 mL mL·∙min−1. The. The particle particle radii radii were weredetermined determined using conventionalusing conventional DLS. DLS. The The displayed displayed particle particle sizesize is measured measured with with the the3D cross 3D cross correlation correlation DLS DLS whilst thewhilst dispersion the dispersion was was pumped pumped through through the the flowflow cell cell at at a adefined defined flow flow rate rateQ betweenQ between 0 and 17 0 and 17 −1 mL·minmL−1.∙min For. each For each flow flow rate, rate, the the presented presented valuevalue for for the the particle particle size size represents represents an average an average of ten of ten DLS measurements (Figure 3, error bars represent standard deviation). DLS measurements (Figure3, error bars represent standard deviation).

(a)

Figure 3. Cont. Appl. Sci. 2018, 8, 108 7 of 10 Appl. Sci. 2018, 8, 108 7 of 10 Appl. Sci. 2018, 8, 108 7 of 10

(b)

Figure 3. Particle size measurement using the DLS with 3D cross correlation geometry of (a) “large” Figure 3. Particle size measurement using the DLS(b) with 3D cross correlation geometry of (a) “large” SiO2 nanoparticles; (b) “small” SiO2 nanoparticles at concentration of 1 wt‐% and flow rates between SiO nanoparticles; (b−)1 “small” SiO nanoparticles at concentration of 1 wt-% and flow rates between Figure2 0 and 3. Particle 17 mL∙min size. measurement2 using the DLS with 3D cross correlation geometry of (a) “large” 0 and 17 mL·min−1. SiO2 nanoparticles; (b) “small” SiO2 nanoparticles at concentration of 1 wt‐% and flow rates between The average particle radius measured using conventional DLS is 157 nm for the large particles 0 and 17 mL∙min−1. and 17.6 nm for the small particles. The average particle size measured using flow cell coupled DLS The average particle radius measured using conventional DLS is 157 nm for the large particles and with 3D modulation is 157 nm for the large particles and 16.2 nm for the small particles. Thus, the The average particle radius measured using conventional DLS is 157 nm for the large particles 17.6 nmmeasurement for the small results particles. obtained The with average conventional particle DLS size and measured DLS with using 3D cross flow correlation cell coupled geometry DLS with 3D modulationand 17.6performed nm is 157 for at nmthe Q = forsmall 0 mL the ∙particles.min large−1 are particles in The excellent average and agreement. 16.2 particle nm for Furthermore,size the measured small particles.both using for the flow Thus, large cell theand coupled measurement small DLS resultswith particles obtained3D modulation the with particle conventionalis 157size nmmeasured for DLSthe using large and DLS particles DLS with with 3D and 3Dcross 16.2 cross correlation nm correlation for thegeometry small geometry remainsparticles. performednearly Thus, the at Q =measurement 0 mLconstant·min− for 1resultsare flow in rates obtained excellent up to 17with agreement. mL ∙conventionalmin−1. Thus, Furthermore, through DLS and the useDLS both of witha for modulated the 3D largecross 3D andcorrelation cross small correlation particles geometry the −1 particleperformedDLS size instrument, measured at Q = 0 mLwe using ∙weremin DLS able are with to in accurately excellent 3D cross analyzeagreement. correlation dispersions Furthermore, geometry comprising remains both differently for nearly the largesized constant silicaand forsmall flow particlesnanoparticles the particle at flow size rates measured up to 17 usingmL∙min DLS−1. with 3D cross correlation geometry remains nearly rates up to 17 mL·min−1. Thus, through the use of a modulated 3D cross correlation DLS instrument, constant forDuring flow the rates synthesis up to 17of mLthe ∙largemin− 1and. Thus, small through silica nanoparticles, the use of a modulatedstarting from 3D the cross molecular correlation we wereprecursor able to accuratelyTEOS, the particle analyze growth dispersions was monitored comprising in real time differently using the sized analysis silica loop nanoparticles connected to at flow DLS instrument, we −were able to accurately analyze dispersions comprising differently sized silica rates upthe to 3D 17 cross mL· correlationmin 1. DLS unit (Figure 4a,b). nanoparticles at flow rates up to 17 mL∙min−1. During the synthesis of the large and small silica nanoparticles, starting from the molecular During the synthesis of the large and small silica nanoparticles, starting from the molecular precursorprecursor TEOS, TEOS, the the particle particle growth growth was was monitoredmonitored in real time time using using the the analysis analysis loop loop connected connected to to thethe 3D 3D cross cross correlation correlation DLS DLS unit unit (Figure (Figure4 a,b).4a,b).

(a)

Figure 4. Cont. Appl. Sci. 2018, 8, 108 8 of 10 Appl. Sci. 2018, 8, 108 8 of 10 Appl. Sci. 2018, 8, 108 8 of 10

((bb)) Figure 4. Measurements during the synthesis process of (a) large silica particles; (b) small silica FigureFigure 4. 4. MeasurementsMeasurements during during the the synthesis synthesis process process of (a) of large (a) largesilica particles; silica particles; (b) small ( bsilica) small particles;particles; the the black black dots dots represent represent real‐timetime 3D3D cross cross correlation correlation DLS DLS measurements measurements in flowin flow (Q (Q= 12.9 = 12.9 silica particles; the black dots represent real-time 3D cross correlation DLS measurements in flow mLmL∙min∙min−1−),1), the the green green trianglestriangles represent conventionalconventional DLS DLS measurements measurements on on the the respective respective non non‐ ‐ (Q = 12.9 mL·min−1), the green triangles represent conventional DLS measurements on the respective movingmoving dispersions dispersions and and thethe redred squares representrepresent TEM TEM measurements measurements (solid (solid concentrations concentrations of silicaof silica non-moving dispersions and the red squares represent TEM measurements (solid concentrations of dispersionsdispersions of of ca. ca. 2.25 2.25 wt wt‐‐%).%). silica dispersions of ca. 2.25 wt-%). ForFor comparison, comparison, also also offoff‐‐lineline DLS measurements on on non non‐moving‐moving dispersions dispersions were were performed performed usingusingFor a astate comparison,state‐of‐of‐art‐art DLS DLS also instrument instrument off-line DLS (Figure measurements 4,4, greengreen triangles). triangles). on non-moving For For that that purpose, purpose, dispersions samples samples were were were performed taken taken usingduringduring a the state-of-art the reaction. reaction. DLS Furthermore, Furthermore, instrument samples (Figure werewere4, green taken taken triangles). for for TEM TEM analysis, For analysis, that purpose,and and immediately immediately samples dried were dried on taken aon a duringTEMTEM grid grid the after reaction.after which which Furthermore, the the samplessamples were samples analyzedanalyzed were (Figure taken(Figure for 4, 4, red TEMred squares). squares). analysis, The The and deviation deviation immediately in particlein particle dried onsizesize a obtained TEM obtained grid using using after DLS DLS which betweenbetween the samples the off‐‐lineline were measuredmeasured analyzed samples samples (Figure and 4and, red the the squares).samples samples measured Themeasured deviation in flowin flow in particleis ismaximum maximum size obtained 7%. 7%. This This using smallsmall DLS deviationdeviation between inin the particleparticle off-line sizesize measured measured measured samples with with conventional conventional and the samples DLS DLS on measured onnon non‐ ‐ moving dispersions and in flow with the 3D cross correlation geometry DLS during synthesis of silica inmoving flow is dispersions maximum and 7%. in This flow small with deviationthe 3D cross in correlation particle size geometry measured DLS with during conventional synthesis DLSof silica on nanoparticles demonstrates that particle growth can be accurately monitored in real‐time using our non-movingnanoparticles dispersions demonstrates and that in flow particle with growth the 3D can cross be correlation accurately geometry monitored DLS in real during‐time synthesis using our of newly developed flow cell coupled DLS setup connected to the reactor using the analysis loop. silicanewly nanoparticles developed flow demonstrates cell coupled that DLS particle setup growth connected can to be the accurately reactor using monitored the analysis in real-time loop. using The solvodynamic radius of the particles during synthesis, obtained through DLS, was our newlyThe solvodynamic developed flow radius cell coupled of the DLS particles setup connectedduring synthesis, to the reactor obtained using through the analysis DLS, loop. was compared to particle size obtained using transmission electron microcopy (Figure 4, red squares, The solvodynamic radius of the particles during synthesis, obtained through DLS, was compared comparedFigure 5). to particle size obtained using transmission electron microcopy (Figure 4, red squares, toFigure particle 5). size obtained using transmission electron microcopy (Figure4, red squares, Figure5).

(a) (a)

Figure 5. Cont. Appl. Sci. 2018, 8, 108 9 of 10 Appl. Sci. 2018, 8, 108 9 of 10

(b)

FigureFigure 5. 5.High High angle angle annular annular dark dark ﬁeld field (HAADF)-STEM (HAADF)‐STEM images images of ( aof) large(a) large silica silica particles particles and (andb) small (b) silica particles. small silica particles.

The average particle size obtained through TEM analyses was determined through The average particle size obtained through TEM analyses was determined through measurement measurement of at least 25 particles per samples. The particle sizes obtained using TEM analyses of at least 25 particles per samples. The particle sizes obtained using TEM analyses were smaller were smaller than those obtained using DLS. This can be explained by the fact that through DLS the than those obtained using DLS. This can be explained by the fact that through DLS the solvodynamic solvodynamic radius of the particle is measured, and through TEM the radius of the dry particles radius of the particle is measured, and through TEM the radius of the dry particles [19]. For the [19]. For the large silica particles, the radii determined through TEM were approximately 10% smaller large silica particles, the radii determined through TEM were approximately 10% smaller than those than those obtained through DLS. For the small silica particles, this difference was significantly larger, obtained through DLS. For the small silica particles, this difference was significantly larger, due the due the strong deviation of these particles from ideal spheres (Figure 5b). In contrast to the DLS strong deviation of these particles from ideal spheres (Figure5b). In contrast to the DLS techniques techniques presented in this article, however, TEM not only provides information on the particle size presented in this article, however, TEM not only provides information on the particle size but also on but also on its shape and architecture. In addition to size, these are also important characteristics that its shape and architecture. In addition to size, these are also important characteristics that influence influence the particle’s property. the particle’s property. 4. Conclusions 4. Conclusions We have successfully developed a flow cell coupled DLS set‐up that allows to monitor the size We have successfully developed a flow cell coupled DLS set-up that allows to monitor the size of of nanoparticles in real‐time during bottom‐up liquid phase synthesis. Through the use of a nanoparticles in real-time during bottom-up liquid phase synthesis. Through the use of a modulated modulated 3D cross correlation DLS instrument, we were able to accurately analyze dispersions 3D cross correlation DLS instrument, we were able to accurately analyze dispersions comprising silica comprising silica nanoparticles in water/alcohol mixtures. Through Stöber synthesis, we prepared nanoparticles in water/alcohol mixtures. Through Stöber synthesis, we prepared small and large small and large silica nanoparticles with a radius of approx. 20 nm and 150 nm, respectively. Both silica nanoparticles with a radius of approx. 20 nm and 150 nm, respectively. Both dispersions could dispersions could be accurately analyzed using our novel flow cell coupled 3D cross correlation DLS be accurately analyzed using our novel flow cell coupled 3D cross correlation DLS set-up at silica set‐up at silica concentrations of approx. 2.25 wt‐% and flow rates up to 17 mL∙min−1. Furthermore, concentrations of approx. 2.25 wt-% and flow rates up to 17 mL·min−1. Furthermore, we demonstrated we demonstrated that the particle size could be accurately monitored in real‐time during Stöber that the particle size could be accurately monitored in real-time during Stöber synthesis of the silica synthesis of the silica spheres starting from TEOS as molecular precursor. This was confirmed by spheres starting from TEOS as molecular precursor. This was confirmed by conventional off-line DLS conventional off‐line DLS and TEM analyses. Thus, the newly developed flow cell coupled DLS set‐ and TEM analyses. Thus, the newly developed flow cell coupled DLS set-up enables the study of up enables the study of nucleation and growth of nanoparticles in bottom‐up liquid phase synthesis, nucleation and growth of nanoparticles in bottom-up liquid phase synthesis, which provides rational which provides rational input for process optimization. Furthermore, when coupled to a production input for process optimization. Furthermore, when coupled to a production sized reactor, it can help to sized reactor, it can help to minimize the number of off spec batches since it provides real‐time minimize the number of off spec batches since it provides real-time information that enables rational information that enables rational process intervention. We expect to validate the set‐up for sol‐gel process intervention. We expect to validate the set-up for sol-gel syntheses of nanoparticles other than syntheses of nanoparticles other than silica (e.g., titania, zirconia) and redox synthesis for the silica (e.g., titania, zirconia) and redox synthesis for the production of metallic nanoparticles starting production of metallic nanoparticles starting from (complex) metal salts. from (complex) metal salts.

Acknowledgments: This work was supported by the European Commission and is part of the EU project COPILOT which has received funding from the European H2020 program under grant agreement n° n◦ 645993. Solliance and the Dutch province ofof NoordNoord BrabantBrabant areare acknowledgedacknowledged forfor fundingfunding thethe TEMTEM facility. facility.

Author Contributions: N.M., P.B. andand M.M. conceived and designed the experiments; R.v.E and R.S. performed the experiments; R.v.E,R.v.E, N.M. N.M. and and P.B. P.B. analyzed analyzed the the data; data; N.K., N.K., R.A. R.A. and and M.V. M.V. contributed contributed to to the the interpretation interpretation of the analyses results; N.M. and P.B. wrote the manuscript. All authors have read and approved the final manuscript. of the analyses results; N.M. and P.B. wrote the manuscript. All authors have read and approved the final Conflictsmanuscript. of Interest: The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

Appl. Sci. 2018, 8, 108 10 of 10

References

1. Black, D.L.; McQuay, M.Q.; Bonin, M.P. Laser-based techniques for particle-size measurement: A review of sizing methods and their industrial applications. Prog. Energy. Combust. Sci. 1996, 22, 267–306. [CrossRef] 2. Buskens, P.; Burghoorn, M.; Mourad, M.; Vroon, Z. Antireﬂective Coatings for Glass and Transparent Polymers. Langmuir 2016, 32, 6781–6793. [CrossRef][PubMed] 3. Mann, D.; Chattopadhyay, S.; Pargen, S.; Verheijen, M.; Keul, H.; Buskens, P.; Möller, M. Glucose-functionalized polystyrene particles designed for selective deposition of silver on the surface. RSC Adv. 2014, 4, 62878–62881. [CrossRef] 4. Segers, M.; Arfsten, N.; Buskens, P.; Möller, M. A facile route for the synthesis of sub-micron sized hollow and multiporous organosilica spheres. RSC Adv. 2014, 4, 20673–20676. [CrossRef] 5. Mann, D.; Nascimento-Duplat, D.; Keul, H.; Möller, M.; Verheijen, M.; Xu, M.; Urbach, H.P.; Adam, A.J.L.; Buskens, P. The Inﬂuence of Particle Size Distribution and Shell Imperfections on the Plasmon Resonance of Au and Ag Nanoshells. Plasmonics 2017, 12, 929–945. [CrossRef][PubMed] 6. Mann, D.; Voogt, S.; van Zandvoort, R.; Keul, H.; Möller, M.; Verheijen, M.; Nascimento-Duplat, D.; Xu, M.; Urbach, H.P.; Adam, A.J.L.; et al. Protecting patches in colloidal synthesis of Au semishells. Chem. Commun. 2017, 53, 3898–3901. [CrossRef][PubMed] 7. Berne, B.J.; Pecora, R. Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics; Courier Corporation: Mineola, TX, USA, 2000; ISBN 978-0486411552. 8. Brown, W. Dynamic Light Scattering: The Method and Some Applications; Clarendon Press: Oxford, NY, USA, 1993; ISBN 9780198539421. 9. Pecora, R. Dynamic Light Scattering Measurement of Nanometer Particles in Liquids. J. Nanopart. Res. 2000, 2, 123–131. [CrossRef] 10. Segrè, P.N.; Vanmegen, W.; Pusey, P.N.; Schatzel, K.; Peters, W. Hard-sphere colloidal suspensions studied by two-colour dynamic light scattering. Opt. Methods Phys. Colloid. Dispers. 1995, 104, 8–11. 11. Block, I.D.; Scheffold, F. Modulated 3D cross-correlation light scattering: Improving turbid sample Characterization. Rev. Sci. Instrum. 2010, 81, 123107. [CrossRef][PubMed] 12. De Kanter, M.; Meyer-Kirshcner, J.; Viell, J.; Mitsos, A.; Kather, M.; Pich, A.; Janzen, C. Enabling the measurement of particle sizes in stirred colloidal suspensions by embedding dynamic light scattering into an automated probe head. Measurement 2016, 80, 92–98. [CrossRef] 13. Urban, C.; Schurtenberger, P. Characterization of Turbid Colloidal Suspensions Using Light Scattering Techniques Combined with Cross-Correlation Methods. J. Colloid Interface Sci. 1998, 207, 150–158. [CrossRef] [PubMed] 14. Khan, M.F.; Dong, H.; Chen, Y.; Brook, M.A. Low Discrimination of Charged Silica Particles at T4 Phage Surfaces. Biosens. J. 2015, 4, 125. [CrossRef] 15. Sato-Berrú, R.; Saniger, J.M.; Flores-Flores, J.; Sanchez-Espíndola, M. Simple method for the controlled

growth of SiO2 spheres. J. Mat. Sci. Eng. A 2013, 3, 237. 16. Ibrahim, I.A.M.; Zikry, A.A.F.; Sharaf, M.A. Preparation of spherical silica nanoparticles: Stober silica. J. Am. Sci. 2010, 6, 11. 17. Stöber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62–69. [CrossRef] 18. Giesche, H. Synthesis of Monodispersed Silica Powders; I. Particle Properties and kinetics. J. Eur. Ceram. Soc. 1994, 14, 189–204. [CrossRef] 19. Meli, F.; Klein, T.; Buhr, E.; Frase, C.G.; Gleber, G.; Krumrey, M.; Duta, A.; Duta, S.; Korpelainen, V.; Bellotti, R.; et al. Traceable size determination of nanoparticles, a comparison among European metrology institutes. Meas. Sci. Technol. 2012, 23, 125005. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).