Synthesis Of Mesoporous Silica Nanoparticles

Published Date: 02 Nov 2017

aDepartment of Physics & Astrophysics University of Delhi, New Delhi-110007,India

*[email protected], Phone no- (91)9711404037, Fax No- 011-23819547

Abstract

Photoluminescence spectra of different concentration of laser grade dye stilbene dye solution and stilbene incorporated in MSN had been studied and compared to determine maximum quantum yield. High quantum yield was obtained for 10-3M concentration stilbene incorporated MSN. This stilbene incorporated MSN silica is shown to give a fluorescence response to gaseous sulphur dioxide, with a response time of a few seconds and fluorescence emission intensity is proportional to the sulphur dioxide concentration, which are desirable properties for gas sensing. These features together make this MSN material very promising for optical sensing applications.

Keywords: Mesoporous nano silica, Fluorescence sensitization, Optical gas sensor

Introduction

Mesoporous materials have been recently proposed as hosts for chemo-sensing molecules due to their highly porous nature combined with low absorption and emission in the visible spectrum makes it excellent candidates for optical sensors [1-4]. Recent developments in mesoporous silica materials provide opportunities to increase the properties of gas sensor devices, since their performance is directly related to the surface area of substances [5-8]. Mesoporous materials are ordered porous materials which display a honeycomb-like structure of uniform mesopores (3 nm diameter) running through a matrix of amorphous silica. They are the result of using surfactant/block copolymer as a template in sol-gel chemistry. The template is removed during the preparation process, while the remaining material has a large tendency to receive gas molecules at the surface. Gas sensors based on mesoporous materials use this advantage to show their characteristic response to the target gas. One interesting property of a mesoporous material, on the other hand, can be its adsorption capacity, which means that a large amount of the material touching its surface may remain attached to the surface until a high temperature desorption process is applied. Optical properties of mesostructured and mesoporous materials grew on the success of dye incorporation [9-14]. The results of dye incorporated MSN shows several advantages, including photostability, sensitivity and also the requirements of fast response and negligible leaching can also be fulfilled for application of gas sensors and nanolasers. Different dyes have been incorporated with in solid matrices to enhance quantum yield and better sensing ability for tunable lasers and sensors [15-18]. Nevertheless, to the authors’ knowledge, no literature has discussed the use of stilbene incorporated MSN as chemical sensing or nanolaser material this attracts attention to explore the opportunity. Stilbene laser dye is an aromatic conjugated diene compounds having tuning wavelength in violet region [19-20]. It is also superior to the known coumarin dyes with respect to threshold, slope efficiency and photochemical stability. It is apposite for many applications including dye lasers because it does not show photochemical degradation even at high pump power levels. Sahare et. al. have developed a gas sensor using stilbene 420 laser dye in ethanol solutions [21]. But, the dye solutions are not suitable for many applications (such as lasers and gas sensors) due to problems of handling and photo stability, which brings change in concentration of dyes over a period of time. The photo stability of dyes can be enhanced by incorporating it in sol-gel matrices [22]. Easwaran et al have discussed briefly increasing photostability of organic dye by molecular incorporation and thus also preventing organic dyes from dimerization [23]

Therefore, in this paper we first reported comparative studies of photoluminescence spectra of stilbene dye solution of different concentration and stilbene dye molecules incorporated MSN. Secondly, the effects of the host on the photophysical, photochemical and sensing properties of stilbene molecules were studied. The objective of the present work is to study the maximum quantum yield of stilbene incorporated MSN and also applicability of it as an optical sensing material with sulphur dioxide.

Optical response of sulphur dioxide with stilbene incorporated MSN based upon fluorescence sensitization. It has been demonstrated that the fluorescence sensitization or increase of fluorescent intensity due to the presence of sulphur dioxide in the stilbene incorporated MSN, could be used to determine the presence of sulphur dioxide gas. Quantitative data on the fluorescence sensitization is reported here. The results obtained will be useful for potential applications in the field of nanosensor and nanolasers. Thus, a sensitive sensor of sulphur dioxide could be constructed but selectivity; sensitivity and reversibility have to be optimized.

2. Material and Methods

2.1 Synthesis of mesoporous silica nanoparticles (MSN)

The siliceous MSN host was synthesized by homogeneous precipitation using tetra ethyl orthosilicate(TEOS) as a silica source and cetyltrimethylammonium bromide as structure directing agent according to a procedure published by Slowing et. al.[24]. Mesoporous nano silica MSN with large pores is synthesized by using pore extending agent (mesitylene) to the Sol-gel method. CTAB (Cetyl Trimethyl Ammonium Bromide) 2.7mmol was dissolved in 480mL of triple distilled deionized water. Then 3.5mL of 2M NaOH in water and 7mL of 48.8mmol mesitylene was added. The mixture was stirred vigorously at 80oC for 2 hour. After 2hour 21.9mmol of TEOS was added drop wise at a rate of 0.5mL/min. The mixture was again stirred for 2hours at 800C. The resulting white precipitate was filtered, washed with abundant ethanol and dried under vacuum at 60oC for 12hour. After drying for 12hours white powder was obtained. Finally the structure-templating CTAB and mesitylene were removed from the composite material via acidic extraction. For this, 1gm of powder was mixed to a solution of 100mL methanol and 0.75mL concentrated hydrochloric acid and stirred for 6hour at 50oC. Thus prepared silica powder was filtered, washed and dried under vacuum at 100oC for 12 hour.

2.2 Dye loading / encapsulation in MSN

Stilbene dye solution of 10-3M-10-6M was prepared by dissolving appropriate amount of dye in 10mL Ethanol. Ethanol solvent is chosen for preparing dye solution because of its solubility and removal at low temperature from MSN without causing degradation of dye on heating. 0.1gm dried MSN was heated at 473K for 6 hour to remove water adsorbed on the surface, then at once transferred to a flask and allowed to cool to room temperature under N2 atmosphere. Five microliter (10-3M-10-6M) stilbene in ethanol solution was added into the flask and stirred for 24hour. The solids were filtered and washed thoroughly with ethanol until the solution was clear, which indicated that the dye molecules attached on the external surface of MSN was removed. Then the samples were dried for photoluminescence studies.

2.3 Characterization of Mesoporous nano silica MSN

2.3.1 Surface Area and porosity measurement

Surface area and pore size distribution of MSN was determined by using the instrument Micromeritics ASAP-2010. For these measurements, MSN samples (with and without stilbene) were degassed at 110 0C for 12 hour and then the nitrogen adsorption–desorption isotherms were obtained. Surface area, cumulative desorption pore volume and micropore volume were determined using Brunauer–Emmet–Teller (BET), Barrett–Joyner–Halenda (BJH) and Dubinin- Radushkevich (DR) methods respectively.

2.3.2 Structural and Morphological characterization

X-ray diffraction spectra were recorded on PAN analytical X’Pert PRO Diffractometer using Ni-filtered CuKα (λ = 1.54056 Å) line. The 2 angles ranged from 1 to 5o for low angle XRD with a speed of 0.050/second. TEM studies were performed to find out the particle size of the MSN. For that 10mg of sample was mixed in 1mL of ethanol and sonicated for 10minutes to achieve a better dispersion of the particles. A drop of supernant was put on carbon coated copper grid and the grid was dried for 24hours before taking the images. The images were taken using TECNAI TEM instrument (FEI TEM model TECNAI G2 T30, U-TWIN) at 300kV. The surface morphology of MSN materials was observed by a field emission scanning electron microscope (FE-SEM) (SEM, JEOL, JSM-6360A). The sample was deposited on a sample holder with an adhesive carbon foil and sputtered with gold.

2.3.3 Photoluminescence studies

Photoluminescence spectra of stilbene dye solution and stilbene incorporated MSN sample was taken using high resolution Varian Cary Eclipse spectrofluorometer equipped with a 15W Xenon flash lamp as a source and wide band PMT as detector respectively. For recording the photoluminescence spectra, the excitation and emission slits were kept fixed at 2.5nm. The scan rate was kept at 600nm/min with data interval of 1nm throughout the experiment.

2.3.4 Photoluminescence studies of Sulphur dioxide with stilbene incorporated MSN

The schematic diagram and detail of the experimental setup is shown in Figure 6. The sensing material used in this experiment is in pellet form of 12mm in diameter. Fluorescence spectrophotometer monitored the fluorescence properties of stilbene incorporated in MSN samples for online examination purpose and the data were recorded by a computer. The scan rate was 600nm/min over the wavelength range from 360 to 600nm. Both sulphur dioxide and nitrogen gases were obtained from gas cylinders. The sulphur dioxide gas concentration was quantified by a Seres make sulphur dioxide analyzer before each test. Then the sulphur dioxide and nitrogen gas streams were mixed in a mixing chamber. All the gas flow rates were controlled by Aalborg make mass flow controller.

3 Results

3.1 Surface area and porosity measurement

Nitrogen adsorption–desorption isotherms and BJH pore size distributions of MSN with and without stilbene have been represented in Figures 1. MSN sample showed higher uptake of nitrogen than stilbene loaded MSN and exhibited hysteresis loops of type H1, which are characteristic of adsorption of nitrogen inside mesoporous structures. This is obvious as the pores size is narrowed on incorporation of the dye molecules inside. BJH pore size distribution curve (as shown in Figure 1) indicated two distinguished peaks. MSN with and without stilbene were found to be having similar type of pore size distributions with peak maxima at 32.5 and 33 Å, respectively (also given in Table 1) along with that some pores having pore diameters around 27 and 25 Å were also observed. Such a high degree of well defined porosity was obtained due to optimized synthesis parameters and mesitylene–CTAB template. The total pore volume (BJH) for MSN was found to be 1.01cc/g and the surface area (BET), 1013 m2/g. As mentioned above, the loading of stilbene dye molecules inside the MSN mesopores has decreased its surface area to 834 m2/g. Micropore size and cumulative desorption pore volume were also found to be decreased on loading stilbene, data is shown in Table 1.

3.2 Material Structural and Morphological investigations

The presence of the ordered mesoporous structure in MSN was also confirmed by high resolution TEM shown in Figure 2a. The spherical shaped particles having size of 200-300nm had been observed and is shown is Fig 2a. Porosity can be clearly seen in the enlarge view of TEM images shown in images Figure 2b. The SAED pattern shown in inset of Figure 2a indicates the MSN is an amorphous phase.

Scanning electron microscopy was performed to illustrate the particle size, surface morphology, and particle size distribution of the MSN. Figure 2c illustrates the SEM images of MSN in small and large domains. The SEM images revealed that MSN has a spherical shape and their size distribution is to some extent large.

The low angle powder X-ray diffraction patterns of MSN and stilbene incorporated MSN are shown in Figure 2d. Strong reflection peak is observed at 2θ value, 1.2(100) together with one small peak at 1.9 (200) degrees has been observed in both Xrd plots and the data is tabulated in Table 2. These reflections are typical of MSN as described by Chen et al. [25]. The Xrd pattern of MSN and stilbene incorporated MSN shows well ordered mesostructure. This indicates that the dye incorporation did not affect the ordered mesoporous structure of MSN also described by Ananthanarayanan et. al [14].

3.3 Photoluminescence spectra

3.3.1 Photoluminescence excitation and emission spectra of stilbene in ethanol

Figure 3 shows the effect of concentration of stilbene dye in ethanol on photoluminescence excitation spectra (keeping emission at 420 nm). Two excitation peaks at 230 and 350 nm are observed for 10-6M dye concentration. The excitation intensity increased with increase in the dye concentration up to 10-5M. But at higher concentrations (10-4M and 10-3M), there is a blue shift observed in these peaks, especially, in the 350 nm peak which then appears around 308 nm. This was plausibly due to the formation of dimers/trimers, which results in quenching and consequently, decrease in the fluorescence intensity [26]. Dimers/trimers and aggregates of higher orders (e.g. tetramers at saturated concentration) could also be formed because as the concentration of dye increases, the dye molecules come closer to form aggregates [27]. These aggregates have different oscillator strengths i.e., singlet–singlet (S0 S1) or higher order transitions, which alter the absorption/excitation spectra. The formation of dimers at higher concentration has been explained by a number of researchers using different type of dyes [28-31]. Exciton theory predicts that the excited-state levels of the monomer split in two upon dimerization [32, 33]. One level is of lower and the other of higher energy than the monomer excited state. This splitting is a consequence of the two possible arrangements (in phase and out-of-phase oscillation) of the transition dipoles of the chromophores in the dimer. The interaction energy between the chromophores is a function of the transition moment of the monomer, the angle and distance between the transition dipoles. Transitions from the ground state to either excited state are possible. However, the number of bands actually observed depends on the geometry of the dimer: (a) for parallel dimers (H-type,) the transition to the lower energy excited state is forbidden and the spectrum consists of a single band blue-shifted with respect to the monomer. (b) For head-to-tail dimers (J-type), the transition to the higher energy excited state is forbidden and the spectrum shows a single band red-shifted with respect to the monomer. The spectrum of a concentrated solution of stilbene of 10-3M, 10-4M which forms both types of aggregates simultaneously, is shown in Figure 3.

Figure 3 also represents the emission spectra of stilbene excited by the respective excitation wavelengths for different concentrations (350, 350, 308, 296 and 268 nm for 10-6M, 10-5M, 4x10-5M, 10-4M and10-3M, respectively). A broad peak at 425 nm with shoulders on both sides, i.e., one at ~400 nm and the other at ~450 nm could be seen here. The fluorescence emissions take place from the triplet–singlet states (T1 S0) after intersystem crossing (e.g. energy transfer from S1 T1) and, therefore, did not alter the emission spectra. The quantum yield depends on the probabilities of such transitions and changes with a change in dye concentration. When the concentration of dye increased from 10-6M to 4x10-5M, the quantum yield increased, however, further increase in dye concentration in the range 10-4M – 10-3M, quantum yield decreased. The quantum yield was the maximum at 4x10-5 M. Similar results are also reported in the literature [26, 34]. This is plausibly due to the formation of dimers/trimers as mentioned by Naik et al[34] who indicated that the dimerization in solutions starts around 10-4M dye concentration with intermolecular distance ~50 nm.

3.3.2 Photoluminescence excitation and emission spectra of Stilbene incorporated MSN

Figure 4 shows the PL excitation spectra (keeping the emission wavelength at 420 nm) for different stilbene dye concentration in MSN. A broad emission spectra with main peak at around 350 nm with shoulders on both sides, (i.e. one at ~285 nm and the other at ~375 nm) has been seen for all dye concentrations. It has been observed here that unlike in case of dye solutions there is no change in the structure of the spectra. Also unlike dye solutions, where the overall intensity of the spectra started decreasing after certain concentration nearly (10-4M), it increased with the dye concentration increasing. Two inferences can be drawn from such a behavior, firstly, there is no strong chemical bonding with the MSN matrix that can alter the excitation spectra with loading of the dye and secondly, the dye molecules are not aggregating to form dimers, trimers and the aggregates of higher orders at higher concentrations due to mesoporous structure.

The emission spectra of stilbene in MSN (for excitation wavelength 350 nm) are as shown in Figure 4. A broad peak at 425 nm with shoulders on both sides, i.e., one at ~400 nm and the other at ~450 nm is clearly seen in the figure. The emission spectra were found similar to that for the stilbene dye in ethanolic solution. As the concentration of stilbene in MSN is increased, the quantum yield increased and unlike in ethanolic solutions, no quenching was observed even at higher concentrations (10-3M) of stilbene incorporated MSN. The observed increase in quantum yield for MSN with stilbene (10-6M–10-3M) arose from a unique spatial distribution of dye molecules inside mesopores of MSN, which keep the dye molecules away from dimerization/aggregation. As a result, higher number of dye molecules (as high as 10-3M) can stay inside without dimerization resulting an increase in quantum yield. Recent reports also indicates the stability and effectiveness of dye monomers encapsulated inside porous nanoparticles (zeolites) [34, 35–37]. The mesopores have a cylindrical geometry (considering that cylindrical nanotube like structures are formed on removal of CTAB and mesitylene molecules through acidic extraction) [37]. The dye molecules are well dispersed within each cylinder along the MSN pore-wall surface. This prevents the dye molecules from dimerization as evidenced from the excitation and emission spectra. It could be observed that the distance between dye molecules inside silica mesoporous particles could be as small as 2–3 nm (i.e. the diameter of a nanopore), which does not allow noticeable dimerization [34, 37].

Therefore based on the photoluminescence results 10-3M stilbene incorporated MSN had been chosen as an active material for gas sensing.

3.3.3 Stilbene incorporated MSN for Sulphur dioxide studies

Fluorescence spectrophotometer monitored the fluorescence properties of stilbene incorporated MSN samples for online sulphur dioxide examination purpose and the data were recorded by a computer. The sensor is based on the fluorescence properties of stilbene incorporated MSN. The fluorescence spectra of the stilbene dye incorporated MSN with varying sulphur dioxide concentrations are as shown in Figure 5. The excitation wavelength was kept at 350nm. It could also be observed from Figure 5 that the nature of the spectra remains almost unaltered after being sensitized by the sulphur dioxide gas. It seems that there is no possibility of a chemical reaction between sulphur dioxide gas and the laser dye stilbene incorporated MSN. The sensitization may therefore be due to collisions of the fluorophore (stilbene) in the mesopores of silica with sensitizing molecules of sulphur dioxide occurring in its excited states. A significant increase in the intensity of the MSN incorporated with stilbene is observed as the concentration of sulphur dioxide is increased. The Stern–Volmer plot for the experimentally observed fluorescence spectra for varying sensitizer concentrations are as shown in inset of Figure 5. It can be seen from this figure that there is a good linear correlation between the normalized intensity and the sensitizer concentration up to around 300ppb.

Fluorescence sensitization refers to a process in which there is an increase in the fluorescence intensity of a certain fluorophore. The material that is responsible for this process is called a sensitizer. Usually a fluorophore which does not form a complex with the sensitizer can exhibit sensitization. The sensitized fluorescence intensity is related to the concentration of the sensitizer. Therefore, the sensitized flurophore can serve as an indicator for sensitizing agent. Basically, fluorescence sensitization or quenching can be described by the Stern–Volmer equation as follows:

Io/I = τo/τ = 1 + Ksv{Q} = 1 + Ksτo{Q}, (1)

where Io and I are the fluorescence intensities in the absence and presence of a quencher, respectively. Ksv is the Sensitizing or quenching constant (Stern–Volmer Constant), {Q} is the Concentration of the sensitizer, Ks is the Bimolecular sensitization constant and τo and τ are the lifetimes of the excited state of the fluorophore in the absence and presence of the quencher or sensitizer.

Equation (1) can be alternatively written as

I/Io = τ/τo = 1 − Ksv{Q} = 1 − Ksτo{Q}.

The values of the Stern–Volmer constant (Ksv) and the bimolecular sensitizing constant (Ks) are determined using the above equation.

To see whether the nature of the emission spectra has been changed on addition of the sensitizer by forming some complex with the dye incorporated MSN, the emission spectra were theoretically deconvoluted to Lorenzian peaks using the following Lorenzian formulation:

I=I0+2A/π W/4(λ-λc) 2+w2 (2)

Where I0 is the initial value of the intensity (baseline offset), I is the intensity at wavelength λ, A is the total area under the curve from the baseline, λc is the peak position (centre of the peak/mean) w2 is sigma (the variance). The deconvoluted peaks were then convoluted and overlapped on the experimental spectrum to see the quality of the fitting. A typical spectral convolution–deconvolution (SCD) of the stilbene dye incorporated MSN emission spectrum is shown in Figure 7. The overlapping of the experimental and theoretically simulated peaks and the resulting residue plot (inset) shows a good quality fitting. Stern–Volmer curves were plotted with varying concentrations of the sensitizer for different peaks (appearing at around 413, 430, 455 and 480 nm) and are as shown in Figure 8. From these curves it can be seen that the shapes of the curves remain almost the same and all of them pass through the same point, indicating that there is no formation of any complexes and the sensitization occurs due to the collision of the fluorophore. The Stern-Volmer constants have also been determined for each curve and are tabulated in Table 3. It can be seen from these data that the values of Ksv are the maximum for the 413nm peak and are the minimum for the 480nm peak. In the experimental spectra the intensity of the main peak or any other peak could not be exactly determined due to the strongly overlapping peaks. Therefore, better correlation of the individual peak could be found if SCD is done. Sensitivity of the sensor probe could be tuned using the SCD method and better sensitivity of the resulting sensor could also be achieved. Sensitivity of the sensor will, however depend upon the power of the source of the incident light, the slit width and the sensitivity of the photo-detector

Conclusion

A sulphur dioxide gas sensing material based on stilbene dye incorporated MSN was proposed. The stilbene MSN sensing material was sensitized after sulphur dioxide gas adsorption. It was demonstrated by fluorescence spectroscopic instrument that the dye incorporated in MSN is a good sensing material which can detect 350ppb level of sulphur dioxide. The sensitivity has been achieved but the selectivity and reversibility of the sensor has to be optimized.

Acknowledgement

Authors are thankful to Dr. Sudershan Kumar, Director, CFEES, DRDO, for providing lab facilities. One of us (SK) is also grateful to CFEES, DRDO for a research fellowship. The financial support provided under the DRDO project (DRDO Project Contract No. CFEES/ATEG/CARS/001/09–10) is also gratefully acknowledged. We also thank the University Science and Instrumentation Centre, University of Delhi, for providing facilities for material characterizations.