Facile and large scale in situ synthesis of the thermal responsive fluorescent SiNPs/PNIPAM hydrogels

Abstract

We herein report the synthesis of fluorescent silicon nanoparticles (F-SiNPs) with uniformly small sizes and stable fluorescence, and further preparation of F-SiNPs/poly(N-isopropylacrylamide) (F-SiNPs/PNIPAM) composite hydrogels by in situ polymerization. This type of hydrogel displayed visible thermal-sensitive phase transition properties. Upon heating, the fluorescence intensity of the hydrogels decreased sharply beyond the lower critical solution temperature (LCST) of PNIPAM. The process of fluorescence quenching and recovery was reversible after repeated heating and cooling cycles. This result suggests that the present organic/inorganic hybrid F-SiNPs/PNIPAM hydrogels will have potential applications in visual detection as fluorescence-based temperature sensors.

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1. Introduction

Currently, studies on silicon nanoparticles (SiNPs) with uniformly small sizes and special fluorescent properties have attracted broad and sustained interest in the fields of chemistry, materials science and nanotechnology.^1–7 The SiNPs can yield strong fluorescence at small sizes (diameters equal to or smaller than 5 nm) because of indirect-to-direct band gap transitions arising from a quantum-size confinement effect. Wide research has demonstrated that the SiNPs exhibit great promise for wide-ranging optoelectronic and biotechnological applications because of their advantageous merits, including their excellent optoelectronic properties, facile surface modification, and biocompatibility, etc.^8–15 In order to achieve the wide-spread applications of SiNPs, synthetic chemists have developed several synthetic methods to prepare fluorescent SiNPs such as solution-phase reduction, laser ablation, mechanochemical methods, microemulsion, and microwave irradiation, etc.^4,16–25 Very recently, He and Lee et al. reported a elegant photochemical strategy which can facilely prepare F-SiNPs in large quantities at a low cost and under mild conditions.^13,26

Embedding colloidal nanoparticles into polymer matrices has proved to be an effective approach to construct functional composite materials. Nano-scale composites based on polymers and functional nanoparticles possess merit in multifunction integration, chemical sensors and optical materials, as well as reinforcing materials.^4,27–31 On the basis of the luminescent properties of nanoparticles and phase transition behaviors of polymers, several composites from responsive polymers and fluorescent nanoparticles have been synthesized and employed as luminescent sensors successfully.³²

Thermo-sensitive hydrogels are sensitive to external temperature stimuli. As a result, they have aroused wide attention over recent years for potential applications in many fields, involving sensors, shape memories, switches and so on.^33–39 Poly(N-isopropylacrylamide) (PNIPAM) is one of the most representative stimulus-responsive polymers. Owing to the existence of the hydrophilic amide groups (–CONH–) and hydrophobic isopropyl (–CH(CH₃)₂) groups on the side chains of the PNIPAM, both linear PNIPAM aqueous solution and cross-linked PNIPAM hydrogels display rapid temperature responses. Specifically, when the temperature is higher than its lower critical solution temperature (LCST), which is about 32 °C in water, the PNIPAM hydrogels will undergo a phase transition from a hydrophilic, water-swollen state to a hydrophobic, globular state, and this kind of phase transition process is reversible.^36,40,41 Among the various reversible and stimuli-responsive gels, fluorescent gels are the most promising because of their excellent and adjustable optical properties.^34,42 We proposed that the combination of stimuli-responsive polymers and fluorescent nanoparticles will endow PNIPAM hydrogels with luminescent emission. This new type of hybrid material will simultaneously possess strong photoluminescence and temperature sensitivity properties, which will result in potential applications in visual detection, especially in biological systems.

In this contribution, we facilely integrated F-SiNPs into a PNIPAM polymer matrix to prepare F-SiNPs/PNIPAM composite hydrogels by in situ polymerization in large-scale quantities. The functionalization of PNIPAM hydrogels by immobilizing F-SiNPs endowed the hydrogel matrix with luminescent properties. The fluorescent properties of the F-SiNPs in thermally-sensitive PNIPAM hydrogels could be affected by the environmental temperature. This type of thermal sensor also exhibited excellent reversibility under repeated temperature changes. Our study suggests that the present thermal responsive composite hydrogels will exhibit potential applications in fluorescence-based temperature sensors.

2. Experimental

2.1 Materials and instrumentation

All solvents and reagents for synthesis were of analytical grade and used without further purification. (3-Aminopropyl)trimethoxysilane (KH540, 97%) was purchased from Aladdin and 1,8-naphthalimide was purchased from Xiya Reagent. N,N-Methylenebisacrylamide (MBA, 97%), N,N,N,N-tetramethylethylenediamine (TEMED) (Biochemicals), ammonium persulfate (APS) (analytical grade) and N-isopropylacrylamide (NIPAM) were purchased from Sinopharm Chemical Reagent Co., Ltd.

UV-vis spectra were recorded on a UV-4820 spectrophotometer (Hitachi, Japan) equipped with exclusive quartz cuvettes at room temperature (20 °C). Photoluminescence spectra were measured on a Fluorescence spectrophotometer (F-4600FL, Hitachi, Japan) equipped with exclusive quartz cuvettes at room temperature (20 °C) (excited wavelength: 340.0 nm; emission wavelength range: 360 to 900 nm; excited slit: 10.0 nm; emission slit: 10.0 nm; PMT voltage: 400 V). Field emission scanning electron microscopy (FE-SEM) was performed on a SU8020 electron microscope (Hitachi, Japan) at 20 kV. Field emission transmission electronic microscopy (FE-TEM) was performed on a JEM-2100F electron microscope (Hitachi, Japan) at 200 kV. Infrared adsorption spectra were recorded on a Nicolet 6700 (KBr disk) Fourier-transform infrared (FT-IR) spectrometer (Thermo Nicolet company, USA). The UV lamp (450 W, 365 nm) was supplied by Qing Da UV Technology Co., Ltd. Dong Guan, Guangdong. The freeze dryer (FD-1B-50) was supplied by Bo Yi Kang Experimental Instrument Co., Ltd. Beijing.

2.2 Preparation of F-SiNPs

Blue-emitting SiNPs were synthesized according to previously reported procedures in the literature.^13,26 Firstly, 1,8-naphthalimide (2 g) was dispersed in Milli-Q water (90 mL). Then KH540 (10 mL) was added slowly. The mixture was thoroughly stirred for about 15 min and the colour of the nanoparticles was monitored by a UV lamp. After UV irradiation, the as-prepared F-SiNPs sample was collected after being allowed to cool naturally to 25 °C. 1,8-Naphthalimide was precipitated through centrifugation at 6000 rpm for 15 min. At this low centrifugation rate, the resultant that remained in clear supernatant would not precipitate. The residual KH540 (MW < 1 kDa) was further removed by dialysis (MWCO, 1000, Bomei/HC148) against Milli-Q water. F-SiNPs with molecular weights larger than 1 kDa were collected and further diluted by Milli-Q water. The purified F-SiNPs aqueous solution was freeze-dried and weighed.

2.3 Preparation of F-SiNPs/PNIPAM hydrogels

The F-SiNPs/PNIPAM composite hydrogels were synthesized facilely by in situ polymerization of NIPAM in the presence of F-SiNPs at room temperature. Specifically, the F-SiNPs powdered sample (0.1 g) was added into Milli-Q water (10 mL). After its complete dissolution, the monomer N-isopropylacrylamide (NIPAM) (1.0 g, 0.009 mol) and crosslinking reagent N,N-methylenebisacrylamide (MBA) (0.03 g, 0.0002 mol) were added to the as-prepared F-SiNPs aqueous solution. After the reaction system had been completely mixed and deaerated using a water vacuum pump, a certain amount of ammonium persulfate (APS, 0.03 g, 0.0001 mol) aqueous solution was quickly added to the above mixture to initiate the polymerization. Finally, the reaction bulb was incubated for 24 h at room temperature. In our experiment, the polymerization reactions were facilely scaled up to 10, 50 and 100 multiples as well, so we could prepare these composite hydrogels in a large scale (see Fig. S1†). For comparison, a pure PNIPAM hydrogel without F-SiNPs was prepared and accelerated by N,N,N,N-tetramethylethylenediamine (TEMED) as well.

3. Results and discussion

3.1 The structure characterization and spectral properties of F-SiNPs

According to the literature, the F-SiNPs were prepared by a simple and cost-efficient method developed by He and Lee et al.^13,26 The size and the dispersibility of the prepared F-SiNPs were characterized by dynamic light scattering (DLS) and field emission transmission electronic microscopy (FE-TEM). According to the DLS measurement shown in Fig. 1(a), the concentration of the F-SiNPs aqueous solution is 0.05 mg mL⁻¹. The average diameter of F-SiNPs is smaller than 3 nm, indicating that the F-SiNPs possess good monodispersity in water. Further high-resolution TEM (HRTEM) images show that the prepared F-SiNPs possess favorable monodispersity with a size distribution ranging from 2 to 4 nm, and all the F-SiNPs display spherical shapes. The HRTEM image clearly displays distinct lattice planes (220) of ∼0.2 nm spacing, revealing the fine crystallinity of the resultant F-SiNPs (see Fig. 1(c)).¹³

Fig. 1 (a) The results of the size distribution of F-SiNPs through dynamic light scattering (DLS) and (b and c) HRTEM images of F-SiNPs.

Optical characterization of the F-SiNPs was then carried out to investigate their luminescent properties. Fig. 2 shows the UV-vis spectrum of the F-SiNPs aqueous solution (10 mg mL⁻¹) at 25 °C, exhibiting a resolved absorption peak at 340 nm. The photoluminescence emission of the F-SiNPs were investigated by exciting the lowest-energy absorption maxima band, which displays a emission peak at 396 nm and exhibits a strong blue fluorescence emission. In addition, the elemental composites of the F-SiNPs were confirmed by the X-ray photoelectron spectroscopy (XPS) spectrum (see Fig. S2†).

Fig. 2 The UV-vis (black) and photoluminescence (red) spectra of F-SiNPs. Inset pictures: the colour of the F-SiNPs under ambient light (left) and 365 nm UV light (right).

3.2 The characterization and spectral properties of F-SiNPs/PNIPAM hydrogels

The scheme in Fig. 3 shows the flow-chart to construct the F-SiNPs/PNIPAM composite hydrogels, according to which, the hydrogels were fabricated by the in situ polymerization of the monomer NIPAM and a little amount of the cross-linking agent MBA in the presence of the radical initiator APS. During the polymerization process, the numerous amino groups on the surface of the F-SiNPs could interact with the amide groups in the PNIPAM polymer networks through hydrogen bonding, which would result in the F-SiNPs being successfully entrapped inside the in situ formed PNIPAM networks.^43–45

Fig. 3 Schematic illustration of the preparative strategy for the thermally responsive fluorescent SiNPs/PNIPAM hydrogels, and their photographs at different temperatures.

In order to gain more understanding about the structure of the hydrogels, The F-SiNPs/PNIPAM composites were then characterized by FT-IR spectra and Field emission scanning electron microscopy (FE-SEM). Fig. S3† displays a comparison of the FT-IR spectra of pure PNIPAM, F-SiNPs, and F-SiNPs/PNIPAM hydrogels. Apparently, these two hydrogels possess similar infrared absorption patterns, indicating that the addition of F-SiNPs does not affect the structure of pure PNIPAM gel. For F-SiNPs/PNIPAM hydrogels, the weak absorption peak near 1030 cm⁻¹ is assigned to the vibrational stretch of Si–O bonding, showing the existence of F-SiNPs in the hydrogel matrix. The field emission scanning electron microscopy (FE-SEM) images show that the freeze-dried F-SiNPs/PNIPAM hydrogels possess a well-defined porous network, and the presence of these interconnected holes gives rise to the typical characteristics of hydrogels (see Fig. S4†). In order to determine the elemental composition of the F-SiNPs/PNIPAM hydrogels, XPS measurements were carried out as well (see Fig. S5†).

3.3 Thermal response of the F-SiNPs/PNIPAM hydrogels

In addition, spectral measurements were carried out so as to investigate the relationship between fluorescence and the thermal responsive behavior of these F-SiNPs/PNIPAM composite hydrogels. Fig. S6† shows the UV-vis measurement of the F-SiNPs/PNIPAM hydrogels, in which a resolved absorption peak near 300 nm can easily observed. The photoluminescent properties were examined by exciting the lowest-energy absorption maxima band at 300 nm, according to the UV-vis absorption. As expected, the fluorescent spectra of the F-SiNPs/PNIPAM composite hydrogels were measured from 20 °C to 50 °C under a gradual temperature change. It could be seen that the fluorescence intensity of the hydrogels decreased abruptly with the increase in temperature in the narrow range of 20–35 °C, and vice versa (see Fig. 4). At low temperature, the hydrogel network is homogeneous. The F-SiNPs are uniformly dispersed in the transparent gel matrix and can display strong fluorescent emission. However, when the temperature is higher than the LCST, the PNIPAM undergoes a phase separation and the hydrogel networks become heterogeneous. The intrachain of the PNIPAM networks collapses and becomes turbid.^44,45 The collapsed PNIPAM particles work as a strong scattering center, resulting in the dramatic decrease of fluorescence intensity of the F-SiNPs, so the fluorescence of the F-SiNPs is suppressed considerably at high temperature.

Fig. 4 Effects of temperature on the PL spectra of the F-SiNPs/PNIPAM composite hydrogels.

It should be noted that our F-SiNPs/PNIPAM hydrogels also exhibit excellent reversibility. Fig. 5 shows the reversible switching of the F-SiNPs/PNIPAM hydrogels between 20 and 50 °C, from which one can learn that the fluorescence intensity of the hydrogels can be readily switched between the two states with excellent reversibility. This result suggests that the present F-SiNPs-based material is an effective thermosensitive sensor. The stability of these F-SiNPs/PNIPAM hydrogels were tested as well. After the composite hydrogels were immersed in deionized water at room temperature for 3 days, the PL spectra of the hydrogels and the aqueous phase were detected respectively. The hydrogels still displayed strong blue fluorescence while no emission could be detected from the aqueous phase (see Fig. S7†). This result demonstrates that the interaction of F-SiNP and PNIPAM is strong enough after in situ polymerization, so that the leaching of the F-SiNPs nanoparticles from the polymer matrix is difficult.

Fig. 5 The fluorescence intensity of F-SiNPs/PNIPAM between low temperature (20 °C) and high temperature (50 °C) over 6 cycles.

4. Conclusions

In summary, a novel type of inorganic/organic hybrid network, F-SiNPs/PNIPAM composite hydrogels, was successfully prepared by using in situ polymerization. The combination of PNIPAM and F-SiNPs successfully endowed the thermal responsive polymer with fluorescent behavior. The composite hydrogels were highly photoluminescent and the fluorescence intensity was reversibly sensitive to external temperature stimuli. The reversible temperature-sensitive fluorescence properties make the material a promising candidate for thermosensitive sensors. We believe that this work has provided a new composite material for the development of F-SiNPs-based thermo-sensitive devices or sensors. Further applications of this composite hydrogels in biological systems are under way.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 21371043 and 21302035), and the Fundamental Research Funds for the Central Universities (2014HGCH0009) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: XPS spectra of F-SiNPs, SEM images and UV-vis spectrum of F-SiNPs/PNIPAM hydrogels. See DOI: 10.1039/c6ra09534h

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