Silica coating of indium phosphide nanoparticles by a sol–gel method and their photobleaching properties
- 92 Downloads
Indium phosphide (InP) nanoparticles face at problems such as aggregation and photobleaching, and formation of core–shell structure is promising to solve the problems. This paper examined the method of coating the InP nanoparticles with silica via sol–gel reaction in reverse micelles. Fabrication of silica-coated InP (InP/SiO2) nanoparticles was performed by using the reverse microemulsion method, in which the sol–gel reaction of tetraethyl orthosilicate and water with ammonia is performed in reverse micelles composed of polyoxyethylene (5) nonylphenylether, branched in cyclohexane containing InP nanoparticles. The InP nanoparticles did not aggregate because of physical barriers of silica shells. The silica coating controlled photobleaching of the InP nanoparticles; the colloid solution of InP/SiO2 nanoparticles emitted more stable fluorescence than that of the shell-free InP nanoparticles, even after the InP/SiO2 nanoparticles were placed in a dark box in an atmosphere set to room temperature.
KeywordsIndium phosphide Nanoparticle Silica Coating Sol–gel Fluorescence
Luminescent semiconductor nanoparticles, which are luminescent quantum dots (QDs), have yet to be examined for their excellent fluorescence properties. In particular, Cd-related QDs, such as CdSe and CdTe, have been studied to develop high-quality light-emitting devices [1, 2, 3]. The Cd-related QDs harm the human body due to toxicity of Cd contained in the Cd-related QDs [4, 5, 6]. Therefore, the use of Cd-free QDs is desired for safety. Apart from Cd-related QD, indium phosphide (InP) nanoparticles, which are one of the Cd-free QDs, have recently been examined as a new luminescent QD [7, 8, 9].
The QDs tend to aggregate based on their small size. Such aggregation makes luminescent QDs come closer, which may weaken their luminescence [10, 11, 12]. Accordingly, luminescent QDs have to be highly dispersed. The formation of core–shell structures prevents aggregation; the shell works as a physical barrier between the core and medium surrounding the particle, which inhibits the core from hitting against other cores after the aggregation of cores. Accordingly, the core–shell formation controls the aggregation of luminescent QDs, which sustains the luminescence of QDs.
The fluorescence of QDs needs to be strong, and its intensity is required to be stable for long periods. The optical stability of the QDs is susceptible to the surrounding environment. Exposure of the QDs to air, solvent, and light might lead to unsteady fluorescence in the consequence of oxidation of their surface, as several researchers have suggested [13, 14, 15, 16, 17]. The formation of the core–shell structure has other functions besides the prevention of particle aggregation, including the prevention of oxidation, because the physical barrier of the shell can keep the QDs from touching oxygen molecules that are contained in the air around them.
Silica is a promising material for shell production because of the chemical stability in many kinds of solvents and its lower toxicity compared with other materials [18, 19, 20]. From this viewpoint, several methods for silica-coating particles, such as metallic nanoparticles, metal oxide nanoparticles, polymer nanoparticles, and QDs, are based on a sol–gel process [21, 22, 23, 24, 25, 26, 27, 28]. Our research group has also performed silica coating of various kinds of particles [29, 30]. In particular, our research group has used silica to coat fluorescent microbeads [31, 32], and the silica coating has been found to have an effect on the abiding stability of fluorescence emitted from the particles. Our previous findings suggest that silica coating provides fluorescence stability for particles other than the fluorescent microbeads. In addition to our works on the fluorescent microbeads, our research group has also performed silica coating of quantum rods (QRs) that are one of the luminescent semiconductor nanoparticles , and the silica coating had the same effect on the fluorescence stability as in the case of the fluorescent microbeads. In our work on the QRs, the QRs were surface-modified with polyethyleneimine (PEI) to disperse the QRs in water, and then the QRs were coated with silica with hydrolysis of tetraethyl orthosilicate (TEOS) in water/ethanol solution. Thus, the process performed in our work on the QRs was also performed for the InP nanoparticles in a preliminary experiment: Surface modification of InP nanoparticles with PEI followed by silica coating of the surface-modified InP nanoparticles was performed preliminarily. However, silica-coated InP nanoparticles (InP/SiO2) were not obtained, though the reason for the unsuccessful silica coating is still unclear. Pietra et al.  have studied the encapsulation of single CdSe/CdS core/shell nanorods in a silica shell by using the reverse microemulsion method, in which a sol–gel reaction is performed in reverse micelles containing the CdSe/CdS nanorods. In the present work, the reverse microemulsion method was extended to the silica coating of the InP nanoparticles. Accordingly, the silica coating performed in the present work is quite different from the silica coating for QRs performed in our previous work: There are differences in fabrication process and chemicals used for silica coating between both the silica coating processes. Therefore, an effect of silica coating on the stability of fluorescence given by the InP nanoparticles may differ from that for the QRs. Since such silica-coating effect has not been investigated for the InP/SiO2 nanoparticles in detail hitherto, the investigation on fluorescence of InP/SiO2 nanoparticles is worth performing for the purpose of practical use of InP nanoparticles.
This article describes silica-coated InP nanoparticles (InP/SiO2) that were created via the sol–gel reaction proceeding in reverse micelles containing InP nanoparticles. The present research focuses on the colloidal stability of particles among various properties, and the aggregation of the InP nanoparticles was controlled by physical barriers of silica shells. The present research also focuses on the photostability of fluorescent materials among various properties, and the photobleaching of the InP/SiO2 nanoparticles was studied by measuring the fluorescence of the colloid solution. As a result, photobleaching of InP nanoparticles was controlled by the silica coating, because diffusion of the oxygen molecules inside the InP nanoparticles was considered to be limited by the silica shells. Details are described as follows.
Figure 1 shows flowchart for the preparation procedure. The silica coating was carried out with a base-catalyzed reaction of TEOS inside reverse micelles of IGEPAL® CO-520 in cyclohexane in a hermetically sealed reactor equipped with a magnetic stirrer at 20 °C. IGEPAL® CO-520, a diluted InP/toluene colloid solution obtained by diluting the as-provided InP/toluene colloid solution with toluene, and TEOS were added in turn to cyclohexane while vigorously stirring. The initiation of the hydrolysis of TEOS was performed by the addition of the ammonium hydroxide solution, which was followed by the condensation of silanol groups generated by the hydrolysis to form a silica shell on the InP nanoparticles. The reaction time, i.e., the silica-coating time, and the temperature were 24 h and 25 °C, respectively. The silica coating was performed at initial concentrations of 8.0 × 10−5 M InP, 7.6 × 10−1 M toluene, 3.4 × 10−3 M TEOS, 2.4 × 10−1 M IGEPAL® CO-520, 1.8 × 10−1 M NH3, and 6.7 × 10−1 M H2O for the InP/SiO2 nanoparticle colloid solution with a lower InP concentration (low-concentration InP/SiO2), and at initial concentrations of 8.0 × 10−4 M InP, 7.6 × 10−1 M toluene, 3.4 × 10−3 M TEOS, 2.4 × 10−1 M IGEPAL® CO-520, 1.8 × 10−1 M NH3, and 6.6 × 10−1 M H2O for InP/SiO2 nanoparticle colloid solution with higher InP concentration (high-concentration InP/SiO2). The InP/SiO2 nanoparticles were washed by treating the reactant solution with centrifugation, removing the supernatant from the solution, adding water to the residual InP/SiO2 nanoparticles, and redispersing the nanoparticles in water. The final InP concentration was adjusted to 10−4 M for the low-concentration InP/SiO2 and to 10−3 M for the high-concentration InP/SiO2 by changing the amount of water during the washing process.
Transmission electron microscopy (TEM) and fluorescence spectroscopy were used for characterization of the samples. The particles were observed with a TEM. The TEM was carried out with a JEOL JEM-2100 microscope operating at 200 kV. The TEM samples were prepared by dropping the nanoparticle colloid solutions on a collodion-coated copper grid and evaporating their dispersants in an atmosphere at room temperature. The average particle size was estimated by measuring the diameters of the dozens of particles and using the volumes calculated from the measured diameters. The particle colloid solutions were placed in a dark box set to room temperature in an atmosphere, and their fluorescence spectra were obtained with fluorescence spectroscopy. The fluorescence spectroscopy was carried out with a Hitachi F-4500 spectrometer using an excitation wavelength of 405 nm.
3 Results and discussion
3.1 Morphology of particles
Figure 2B shows the photograph of a high-concentration InP/SiO2 nanoparticle colloid solution. The colloid solution had the color of pale orange. Because the color could be attributed to the color that was derived from the raw InP colloid solution, the presence of InP nanoparticles was confirmed by increasing the InP concentration. Figure 2b shows the TEM image of a high-concentration InP/SiO2 nanoparticles contained in the colloid solution. Quasi-spherical particles consisting of InP nanoparticles as core and SiO2 as shells were produced, as well as the case of the InP concentration as low as 10−4 M, and the average size of the InP/SiO2 core–shell nanoparticles was 39.7 ± 5.3 nm.
The present research focuses on the colloidal stability of the InP nanoparticles, as described in the Introduction. The aggregation of the InP nanoparticles was controlled by physical barriers of silica shells, which expected that deterioration of luminescence property of the InP nanoparticles based on particle aggregation would not take place. The above-mentioned results on the dependence of the particle morphology on the ammonia concentration implied that various preparation parameters, such as the surfactant concentration, water concentration, and temperature, should also affect particle morphology, which suggests that InP/SiO2 core–shell nanoparticles with a greater InP concentration can be produced by the performance of optimized preparation conditions.
3.2 Fluorescence properties
The high-concentration InP/SiO2 nanoparticle colloid solution was used for discussion on its fluorescence properties, because it exhibited strong fluorescence, which made it easy to measure its fluorescence intensity.
The present work examined a method for stabilizing the fluorescence of indium phosphide (InP) nanoparticles. The method revolved around the formation of silica shells on the InP nanoparticles, that is, the InP/SiO2 nanoparticles. The silica coating was based on the sol–gel reaction of tetraethyl orthosilicate and water with ammonia in the reverse micelles composed of polyoxyethylene (5) nonylphenylether, branched that formed in cyclohexane and contained the InP nanoparticles. The execution of the sol–gel process fabricated the silica shells on the InP nanoparticles or produced a core–shell structure composed of the InP nanoparticle core and the silica shells, meaning that aggregation of InP nanoparticles was controlled with the silica coating. The InP/SiO2 nanoparticle colloid solution exhibited steady fluorescence compared with that of the silica shell-free InP nanoparticles.
This research was partially supported by Merck Performance Materials Ltd.
Compliance with ethical standards
Conflict of interest
The authors declare no conflicts of interest.
- 4.Modlitbová P, Pořízka P, Novotný K, Drbohlavová J, Chamradová I, Farka Z, Zlámalová-Gargošová H, Romih T, Kaiser J (2018) Short-term assessment of cadmium toxicity and uptake from different types of Cd-based quantum dots in the model plant Allium cepa L. Ecotoxicol Environ Saf 153:23–31CrossRefGoogle Scholar
- 29.Kobayashi Y, Gonda K (2017) The development of quantum dot/silica particles for fluorescence imaging and medical diagnostics. In: Klein L, Aparicio M, Jitianu A (eds) Handbook of sol–gel science and technology—processing, characterization and applications, 2nd edn. Springer, Berlin, pp 1–38Google Scholar
- 31.Kobayashi Y, Misawa K, Kobayashi M, Takeda M, Konno M, Satake M, Kawazoe Y, Ohuchi N, Kasuya A (2004) Silica-coating of fluorescent polystyrene microspheres by a seeded polymerization technique and their photo-bleaching property. Colloids Surf A Physicochem Eng Asp 242(1–3):47–52CrossRefGoogle Scholar
- 35.Mushtaq I, Daniels S, Pickett N (2008) Preparation of nanoparticle materials. US patent 0160306Google Scholar