Manufacture of glass nanoparticles by electrospraying

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2013 J. Micromech. Microeng. 23 025023

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IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 23 (2013) 025023 (9pp) doi:10.1088/0960-1317/23/2/025023

Manufacture of glass nanoparticles byelectrosprayingKazuhiro Uchida1, Atsushi Hotta, Koichi Hishida and Norihisa Miki

Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama,Kanagawa 223-8522, Japan

E-mail: uchida@a3.keio.jp

Received 8 September 2012, in final form 5 December 2012Published 21 January 2013Online at stacks.iop.org/JMM/23/025023

AbstractGlass substrates functionalized by biochemical substances and/or metal thin films have beenused in a number of micro-total analysis systems (μTAS) and microelectromechanical systems(MEMS) devices. We propose a dry patterning process for glass nanoparticles (NPs) using anelectrospray of the sol of tetraethyl orthosilicate (TEOS) containing hydrochloric acid as acatalyst. We experimentally found that the size of the glass NPs was controlled by the viscosityand feed rate of the TEOS sol, and the applied voltage. In order to verify the usefulness ofthese glass NPs, we deposited silver NPs on the glass NPs using a modified silver mirrorreaction. Silver NPs are reported to enhance the Raman scattering, which is required forultrasensitive biochemical sensing. Silver NPs on the glass NPs were experimentally found toexhibit greater surface-enhanced Raman scattering than those on a flat glass substrate. SilverNPs can be used in chemical sensors, such as surface-enhanced Raman scattering (SERS) andfluorescence spectroscopy, due to the enhanced electromagnetic field on the surface. SilverNPs are deposited on the glass NPs by the silver mirror reaction with dispersants, forapplication as ultrasensitive sensors. When silver NPs are formed sterically congested, theenhanced Raman spectrum from the silver NPs on the electrosprayed glass NPs shows anintensity three times that from silver NPs on a flat glass plate substrate. The glass NPs formedby electrospraying are thus proving to yield high performance substrates for chemical sensors.

(Some figures may appear in colour only in the online journal)

1. Introduction

Nanoparticles (NPs) possess a wide variety of attractivecharacteristics originating from their extremely small sizes.Nanoscale structures are known to display structure-basedcolors in nature [1–3]. Various structural colors wereartificially created by tuning the gaps of distributed magneticNPs by an external magnetic field and immobilizing them in aphoto-curable polymer, which was named ‘M-ink’ [4]. MetalNPs, such as silver and gold, have been applied to ultrasensitivechemical/biological detection by means of fluorescenceenhancement [5–10] and surface-enhanced Raman scattering(SERS) [11–15]. The extremely large surface/volume ratioof NPs augments the efficiency of a surface chemical

1 Author to whom any correspondence should be addressed.

reaction. Highly efficient catalytic nanoparticles have thusbeen developed [16–18].

For manufacturing NPs, given their small sizes of10–100 nm, bottom-up techniques such as liquid-phaseprocesses [15, 19–22] are more attractive than top-downtechniques such as photolithography and e-beam lithography,due to the shorter process time and lower costs. For example,citrate reduction of gold chloride is one of the most frequentlyused techniques to form gold NPs [15, 19–22].

Glass NPs can potentially augment the performance ofvarious micro-total analysis systems (μTAS) devices. Glassis used as the substrate material for chemical and biologicalexperiments owing to its high chemical resistivity, mechanicalstrength, transparency and low cost. Glass surfaces can evokespecific chemical reactions, such as the silver mirror reactionthat can deposit silver onto glass surfaces in thin film orNP forms [10, 15]. Bio/chemical materials, such as proteins

0960-1317/13/025023+09$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA

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and DNA, can be attached to the surfaces of glass substratesdirectly or via binding chemicals for analytical use [23–25].Substrates covered by glass NPs have far greater surface areasthan flat glass substrates and are considered to enhance theefficiency of the abovementioned reactions. Other applicationsof glass NPs include enhancement of mechanical strength suchas reinforcing artificial bone [26], increasing capacitance [27]and controlling optical characteristics [28].

Liquid phase or gas phase processes have beeninvestigated for forming glass NPs. The liquid phase processesuse sol–gel synthesis. They are carried out at low temperaturesand atmospheric pressure. The size of the particles can becontrolled by the process parameters, which include solutionconcentration and shear viscosity [29]. Synthesized silica NPscan be coated on substrates by spin coating or dip coating[30]. By spatially modifying the surface, areas where NPs aredeposited can be patterned [31]. When liquid phase processesare used to deposit glass NPs on micro channels of μTASdevices, a concern is that the chemicals used in the liquid phaseprocesses may remain or damage the bio/chemical samplespatterned in the channels in advance, which deteriorates deviceperformance. Hence, dry gas phase processes are preferable insome applications. Gas phase processes include laser ablationof Bi2O3-based erbium-doped glass material [32], nontransfer-arc linear plasma [33] and combustion synthesis using amultielement diffusion flame burner [34]. These processes aredry but are conducted either in a high vacuum or at a hightemperature.

Electrospraying and electrospinning have been widelyused to form nano-fibers of polymer. A high electric field isapplied between the substrate and the nozzle, from wherecharged polymer solution in the nozzle is drawn. When theconcentration of the solution is low, nanoparticles are pro-duced. This process is called electrospraying. Electrospinningproduces nanofibers from the high concentration solution.Note that this is a one-step process and substrates are kept dryduring the process. The areas onto which NPs are depositedcan be determined by a stencil mask [35].

In this paper, we propose and characterize glass NPdeposition using an electrospray of tetraethyl orthosilicate(TEOS). Sol–gel synthesis of TEOS takes place either betweenthe nozzle and the substrate or on the substrate, and glassNPs are subsequently formed on the substrate surface. Thisprocess can be conducted at room temperature and atmosphericpressure under dry conditions. We experimentally investigatedthe NP size as a function of the viscosity of the TEOS solution,feeding rate of the solution and applied voltage between thenozzle and substrate. Spherical glass NPs on the order of100 nm diameter were successfully deposited. In addition,we demonstrated that glass NPs can be used as a substrate forsilver NPs deposition. The substrate exhibited greater surface-enhanced Raman scattering compared to silver NPs on a flatglass substrate.

Figure 1. Electrospraying of glass NPs.

2. Materials and methods

2.1. Materials

Precursor TEOS sol was prepared with the composition ofTEOS (36 mL), deionized water (20 mL), ethanol (58 mL)and hydrochloric acid (HCl) (1.0 M, 4 mL). TEOS, ethanoland HCl were all purchased from Wako Chemicals Co. HClworked as a catalyst to promote sol–gel synthesis of TEOS.

The silver mirror reaction solution contained aqueoussilver nitrate (5 wt%, 5 mL), ammonia aqueous (1.5 M,8 mL), deionized water (20 mL) and disperbyki-102 (1.5 mL)that worked as a dispersant [15]. Without this dispersant, asilver film was formed on the glass surface in the followingreduction process. Hydrazine (6 mL) was added as the reducingagent to initiate silver NPs deposition. Silver nitrate, ammoniaand hydrazine were purchased from Wako Chemicals Co.,and disperbyki-102 was purchased from BYK Japan Co.Rhodamine 6G (R6G), which is typically used as an analytefor SERS and fluorescence spectroscopy, was also purchasedfrom Wako Chemicals Co.

2.2. Electrospraying of glass NPs

During sol–gel synthesis, TEOS was hydrolyzed to becomeSi(OH)4 and subsequently, SiO2. The TEOS sol was stirred atroom temperature for 10 min. Gelation of the sol did not takeplace at room temperature over a month. After the chemicalswere mixed, the sol was heated at 70 ◦C to promote gelation.Progress of gelation as a function of time determined theviscosity of the precursor sol. The viscosity of the gel wasmeasured using an ARES G2 rheometer (TA instruments) at20 ◦C.

Figure 1 shows a schematic diagram of the electrosprayingequipment. A glass syringe (Terumo CO.) with a nozzlediameter of 0.4 mm was used in the experiments. It was placed

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horizontally at a height of 12 cm from the ground. The distancebetween the nozzle and silicon substrate was set to 40 mm. Thesyringe was filled with the sol. Air trapped inside the syringewas carefully removed. When a high voltage on the orderof kV was applied between the nozzle and the substrate, thesol formed a Taylor cone at the tip of the nozzle. A minuteamounts of the sol was separated from the Taylor cone andflew toward the substrate. During the flight the droplets brokeup into smaller droplets and, given the large surface to volumeratio, the solvents and water were removed quickly from thedroplets. The droplets formed glass NPs when they reachedthe substrate. We measured the mean and standard deviationof the diameters in an area of 2.4 μm × 1.7 μm from FE-SEM(HITACHI S-4700) images. The data were taken from threepoints for five substrates. For calculating the areas coveredwith the glass NPs, ImageJ software was used.

The size of the glass NPs is considered to be determinedby the gelation ratio of the sol, feed rates and applied voltagebetween the nozzle and the substrate. The gelation ratio canbe evaluated by the viscosity. The sol was stored for 0 to 24 hat 70 ◦C and the viscosity was measured. The sol with variousviscosities was electrosprayed at a feed rate of 1.0 mL h−1,an applied voltage of 5 kV and the volume of 0.05 mL.Based on the experiment with respect to the gelation ratio,in the following experiments, a sol of 7 mPa s was used,and the experiment was conducted at various applied voltagesranging from 2.5 kV to 12.5 kV for optimization at a feedrate of 1.0 mL h−1, with the volume of the sol being 0.05 mL.Finally, the feed rate was changed to 0.25, 0.5, 1.0, 1.5 and2.0 mL h−1 to observe the dispersion when the applied voltagewas 6.2 kV and the sol volume was 0.05 mL.

2.3. Application of glass NPs to SERS

SERS is known to be very sensitive to the nanoscale surfacemorphology of a metal structure. Therefore, we decided tocontrol the amount of glass NPs to be deposited and forma three-dimensional substrate for silver NPs deposition bycontrolling the amount of precursor TEOS solution. Wedeposited silver NPs on the glass NPs manufactured fromthe TEOS sol solution of 0.01, 0.05, 0.3 and 1.0 mL volume.The viscosity of the sol, the applied voltage and the feed ratewere 7 mPa s, 6.2 kV and 1.0 mL h−1, respectively. The siliconsubstrates on which the glass NPs were deposited were cleanedwith an oxygen plasma. The substrate and hydrazine, whichworks as a reducing agent, were immersed into the silver mirrorreaction solution that was stirred at 25 ◦C. After 80 s, thesubstrate was picked up, rinsed with deionized water and driedusing nitrogen gas. The substrates were observed by FE-SEM.As references, we prepared a pristine glass substrate (S9111Matsunami Glass Ind. Ltd) and a glass substrate covered withsilver NPs. These NPs were deposited by the modified silvermirror reaction for 80 s, which has been reported to exhibithigh SERS [15].

R6G with a concentration of 100 μM was dispensed ontothe substrates: a pristine glass substrate, the glass substratewith silver NPs and the silicon substrate with glass NPscoated with silver NPs. SERS measurements were carriedout using a CCD Raman system STR 300-3LCCD RAMAN

Figure 2. Viscosity of the TEOS sol as a function of storage time at70 ◦C.

(SEKI TECHNOTRON Co.). The laser used by the systemhad a wavelength of 532 nm and the integration time was 3 s.Five points were randomly selected on each substrate for themeasurements.

3. Experimental results

3.1. Effect of viscosity of the sol

The relationship between the storage time and viscosity ofthe sol is shown in figure 2. The viscosity increased with thestorage time from 3 to 60 mPa s. After 21 h of storage at70 ◦C, gelation was observed to progress rapidly; the viscositycould not be measured by an ARES G2 rheometer since thesol turned into a non-Newtonian fluid.

Electrospraying of the sol with viscosities of3 mPa s (without storage at 70 ◦C), 7 mPa s (15 h),10 mPa s (17 h), 20 mPa s (19 h), and 60 mPa s (21 h)was conducted. The appearance and SEM images (100 × and10 000 × magnification) of the substrates are shown infigures 3(a)–( j). The sol with 3 mPa s viscosity did not yieldNPs while the residues of large droplets were observed onthe substrate, as shown in figure 3( f ). It is considered xthatthe sol in this case was not viscous enough so that Taylorcones were not created when a voltage of 5 kV was appliedbetween the nozzle and the substrate. Therefore, large dropletsof the sol were pulled out from the nozzle and flew to thesubstrate without allowing water and ethanol to evaporateduring the flight and the droplets evaporated on the siliconwafer. When the sol with 7–60 mPa s viscosity was used,the substrate looked white, covered by glass NPs as shown infigures 3(b)–(e), different from figure 3(a). The SEM photosshow that the sizes and the distribution of the deposited glassNPs depended on the viscosity of the precursor sol. When theviscosity was greater than 10 mPa s, as seen in figures 3(i)and ( j), particles greater than 1 μm in diameter were createdalong with smaller NPs and the size distribution was found tobe large. Figure 3(g)–(l) show the range of the size dispersionat each viscosity. When the viscosity was 7 or 10 mPa s,particles were found in a small range, where the center of thedistribution was 100–150 nm. We considered that 7 mPa s wasthe optimum viscosity of the sol for electrospraying.

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(a) (b) (c) (d) (e)

( f ) (g) (h) (i) ( j)

(k) (l )

(m) (n)

Figure 3. (a)–(h) Photographs of the substrates after electrospraying of the sol with viscosities of (a) 3 mPa s, (b) 7 mPa s, (c) 10 mPa s,(d) 20 mPa s and (e) 60 mPa s. SEM micrographs of glass NPs created using sol with viscosities of ( f ) 3 mPa s, (g) 7 mPa s, (h) 10 mPa s,(i) 20 mPa s and ( j) 60 mPa s. (k)–(n) Histograms of the size distributions of the glass NPs for sol viscosities of (k) 7 mPa s, (l) 10 mPa s,(m) 20 mPa s and (n) 60 mPa s.

3.2. Applied voltage and feed rates of the sol

Figure 4 shows SEM micrographs and the size distributionof the glass NPs that were electrosprayed at various voltages,when the viscosity of the precursor TEOS sol was 7 mPa sand the feed rate of the sol was 1.0 mL h−1. When the appliedvoltage was 2.5 kV, a small number of glass NPs was formed,as shown in figure 4(a), or no particles were formed. Therefore,

we consider that a voltage greater than 5 kV is requiredto electrospray the TEOS sol. The glass NP sizes and sizedistribution increased as the applied voltage increased. Thehigher voltage generated a greater electric field between thenozzle and the substrate, which culminated in greater flyingspeeds of the sol droplets and thus shorter times of flight thatdid not allow water and ethanol to evaporate and/or dropletsto break up. Voltages of 5 and 6.2 kV were found to be suitable

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(a) (b) (c)

(d) (e) (f )

(g) (h) (i)

(j) (k) (l)

Figure 4. (a)–( f ) SEM micrographs of glass NPs created by applying voltages of (a) 2.5 kV, (b) 5.0 kV, (c) 6.2 kV, (d) 7.5 kV, (e) 10.0 kVand ( f ) 12.5 kV. (g)–(l) Histograms of the size distribution of glass NPs at (g) 2.5 kV, (h) 5.0 kV, (i) 6.2 kV, ( j) 7.5 kV, (k) 10.0 kV and(l) 12.5 kV.

to manufacture glass NPs that yielded glass NPs with smallsizes and good uniformity.

SEM micrographs of the glass NPs are shown in figure 5when the viscosity of the sol was 7 mPa s, the applied voltagewas 6.2 kV, the supplied TEOS sol volume was 0.05 mL, andthe feed rate was varied. When the feed rates were as low as0.25 and 0.50 mL h−1, glass NPs aggregated to form three-dimensional structures. Higher feed rates produced a smallernumber of glass NPs. The size and size distribution of theglass NPs did not depend on the feed rates of the sol. Figure 6shows that large areas of the silicon substrate surface werecovered with the electrosprayed glass NPs. A higher feed ratedecreased the areas covered with glass NPs. Due to the fastfeed rate, a small portion of the supplied solution might dropfrom the tip of the nozzle before the Taylor cone was formed

or when the Taylor cone was unstable. Compared to the fastfeed rate, most of the supplied solution might transform intoglass NPs when the solution was fed slowly. This informationwill be helpful in designing a substrate for sensor applicationswhere the surface areas affect performance.

3.3. Silver deposition and enhanced Raman spectra

Figures 7(a)–(d) show SEM micrographs of glass NPsproduced from the TEOS sol solution with volumes rangingfrom 0.01 to 1.0 mL, when the viscosity of the sol, theapplied voltage and the feed rate were 7 mPa s, 6.2 kVand 1.0 mL h−1 respectively. Figures 7(e)–(h) show SEMmicrographs of silver NPs deposited onto glass NP substrates.The silver mirror reaction takes place on a glass surface. For

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(a) (b) (c)

(d) (e)

Figure 5. SEM micrographs of glass NPs formed with feed rates of (a) 0.25 mL h−1, (b) 0.5 mL h−1, (c) 1.0 mL h−1, (d) 1.5 mL h−1 and(e) 2.0 mL h−1.

Figure 6. Proportions of the surface area of silicon substratescovered with glass NPs as a function of the feed rate.

example, in Kurooka’s work, when the surface of glass iscovered by octadecyltrichlorosilane (OTS), no particles weredeposited [10]. We compared the SEM micrograph before(figures 7(a)–(d)) and after (figures 7(e)–(h)) the silver mirrorreaction. We consider that the change in the geometry wascaused by the silver particles. At a volume of 0.01 mL,glass NPs were formed discretely (figure 7(a)) and silver NPswere deposited uniformly on the silicon substrate while theywere deposited with a small preference onto the glass NPs(figure 7(e)). For a volume of 0.05 mL, some aggregates ofglass NPs were observed (figure 7(b)) and silver NPs werepreferentially deposited onto the glass NPs (figure 7( f )). At0.3 and 1.0 mL amounts, the glass NPs aggregated and formedthree-dimensional nanostructures (figures 7(c) and (d)) andsilver NPs were also deposited onto the three-dimensionalglass nanostructures (figures 7(g) and (h)).

Figure 8 shows the enhanced Raman spectrum of R6Gfrom each substrate. It was reported that enhancement ofRaman scattering had the optimum structures of the silverNPs [15]. In prior work, it was reported that 80 s of silverNP deposition was the best whereas deposition longer than80 s resulted in a lower SERS effect in spite of the presenceof a large amount of silver NPs and three-dimensionalaggregates of silver NPs. The silver NPs on a flat glasssubstrate exhibit a SERS effect as shown in figure 8. Theenhancement factor, which is the ratio of the intensity from theSERS substrate to that from the pristine flat glass substrate,was reported to be 106 [10]. Silver NPs deposited onto glassNPs from 0.01 mL sol were found to exhibit a SERS signalthree times as high as from the NPs deposited onto a flatglass substrate. One of the reasons for this is consideredto be the larger surface areas of the glass NP substrate. Onthe other hand, the silver NPs formed on the glass NPsfrom 0.05, 0.3 and 1.0 mL sol exhibited lower SERS signalseven though they had larger surface areas than that fromthe 0.01 mL sol. According to Kurooka et al, when silverNPs are aggregated too much, the enhanced Raman spectrumis weaker than when the silver NPs are arranged tightly[10], indicating that the SERS had the optimum structuresof silver NPs. In this case, 0.01 mL substrate was consideredas the best substrate for this sensor. When 0.01 mL of solwas sprayed, the glass NPs are deposited sparsely and theywork well as a substrate. The 0.01 mL substrate successfullyenhanced the surface areas of silver NPs while maintaining theoptimum distribution of the silver NPs. These experimentalresults verified and confirmed the usefulness of the glassNPs substrate that can be controlled by the electrosprayingconditions.

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(a) (b) (c) (d)

(i) (j) (k) (l)

( e) (f) (g) (h)

Figure 7. The different structures obtained for different amounts of sol supplied. (a)–(d) SEM micrographs of glass NPs at each suppliedamount of sol solution, (e)–(h) SEM images of silver NPs on glass NPs and (i)–(l) schematics of the corresponding nanoscale structures.

Figure 8. Enhanced Raman spectra obtained for each substrate.

4. Conclusions

We have proposed deposition of glass NPs usingelectrospraying of TEOS sol, comprising TEOS, ethanol,deionized water and HCl as a catalyst. The electrosprayingprocess was experimentally characterized and the formed glass

NPs were found to depend on the viscosity of the precursorsol, the applied voltage and the feed rate. The viscosity ofthe sol could be controlled by the storage time at 70 ◦Cwhile the catalytic reaction progressed in the sol. When thenozzle diameter was 0.4 mm and the distance between thenozzle and the substrate was 40 mm, the smallest viscosity

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and the lowest voltage to create glass NPs were 7 mPa sand 5 kV, respectively, when the smallest NPs with thebest monodispersity were formed. The feed rate of the soldetermined the distribution and aggregation of the NPs onthe substrate. More aggregation was found at the smaller feedrate. We used the glass NPs as the substrate for silver NPdeposition, which was applied to surface-enhanced Ramanscattering (SERS). The silver NPs deposited onto the glassNPs produced by 0.01 mL of sol were found to exhibit higherSERS than those deposited onto a flat glass substrate. SinceSERS depends not only on the surface area but also onthe surface morphology, greater amounts of glass NPs thanthose from 0.01 mL sol did not contribute to the SERS.Appropriate three-dimensional glass surfaces depending onthe application can be created by electrospraying. Thedry, room-temperature and atmospheric electrospraying ofglass NPs proposed herein is readily applicable to augmentperformance of MEMS and micro-TAS devices.

Acknowledgment

This work was supported by a grant-in-aid for ScientificResearch (S) (2122606).

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