Salicylideneanilines encapsulated mesoporous silica functionalized gold nanoparticles: a low temperature calibrated fluorescent thermometer

Abstract

In this study, a novel temperature responsive fluorescent sensor, 4-(2-hydroxybenzylideneamino)benzoic acid (HBA), encapsulated in the nanochannels of mesoporous silica functionalized with gold nanoparticles (GMS) was synthesized and studied. The fluorescence intensity of HBA–GMS showed excellent linear temperature sensitivity over a wide range, from cryogenic to room temperature (100–298 K). Meanwhile, GMS was used as an immobilization matrix to improve light harvesting and calibrate the HBA fluorescence intensity at different temperatures because of the stable and insensitive fluorescence signal of the gold nanoparticle intercalated into the walls of GMS. In addition, it was found that HBA–GMS exhibits excellent biocompatibility and low toxicity for cellular imaging due to the robust GMS support. These results suggest that the assembled mesostructure provides a promising, intelligent, and calibrated fluorescent thermometer with potential applications as a sensor and in cryogenic bio-detection and therapy fields.

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Introduction

Temperature is a very important measured physical property in industrial and scientific fields. Among the numerous methods to determine temperature, fluorescence measurement sensors have attracted much attention in the past few years, for they are accurate and have easily observable detection and more sensitive signals.^1,2 Recently, some temperature-sensitive fluorescent material sensors, such as nanotube-based systems, quantum dots, and organic dyes, have been used to measure temperatures over a wide range.^3–9 Particularly, organic molecules are attractive candidates to sense temperature, especially because of their broad excitation profiles, large absorption cross sections, and tunable emission energies.^10–14 However, the development of efficient fluorescence-based temperature sensors with a stable signal over a wide range of temperatures, especially in ultra-low temperatures, remains a critical scientific challenge.

Recently, salicylideneaniline and its derivatives have been reported and these exhibited thermochromism and photochromism in the solid state. This phenomenon can be explained by taking account of the photo- and thermally induced changes of the fluorescence.^15–20 Most salicylideneanilines exist mainly as the cis-keto form at room temperature in the solid state. However, the population of the cis-keto form decreases and that of the enol form increases with decreasing temperature. Furthermore, the excited S₁ state will generate a hot cis-keto* vibrational state. The hot cis-keto* form in the S₁ state may easily convert to the cis-keto* form in the S₁ state because the hot cis-keto* form will have higher vibrational energy. Hence, the cis-keto* and hot cis-keto* forms are two vibrational states influenced by temperature, following the Boltzmann distribution law.^21,22 Thus, we firstly chose a salicylideneaniline with two reverse cis-keto tautomers as a thermally responsive fluorescent candidate for thermometers over a wide temperature range.

On the other hand, although numerous organic molecules have been reported with intense fluorescence, common organic molecules have two important drawbacks, including rather poor optical signal stability and high toxicity.^23,24 Common fluorescence systems could not be used in both solid and solution systems due to the solubility of the probes, which is affected by their microenvironments and is easily impaired or even lost.^25,26 In addition, the potential toxicity of organic dyes, in that they could have undesirable dangerous interactions with biological cells and the potential for generating toxicity, has been reported. Thus, the chemical stability and toxicity of organic materials will have to be investigated to ensure that they are safe for medical applications. More importantly, the optical signals from the temperature sensors ideally should be stable and undisturbed under complex biological conditions.²⁷ To overcome these drawbacks and to avoid disturbance from their surroundings, an appropriate approach involving encapsulation has proved to be effective.^27–29 Therefore, it is vital to pursue ideal and suitable supports to encapsulate organic molecules to improve their optical stability and biocompatibility as well as their ratiometric fluorescence signal. Furthermore, dual emission fluorescence systems have also been developed to improve ratiometric fluorescence signals to avoid disturbance from optical signals from the surroundings. Recently, some papers have reported the dual emissions of polymer dots, semiconductor nanocrystals and molecular beacons as temperature insensitive calibration parts.³⁴ In this study, we have designed the first ratiometric temperature sensors with two emission peaks using gold nanoparticles for the calibrated fluorescence part.

Recently, using mesoporous silica as scaffolds has been receiving greater attention due to their well-ordered mesoporous structures, large pore diameters and high surface areas, which all facilitate the possibility of encapsulating and delivering large quantities of small molecules.^30,31 In particular, some groups have recently reported a mesoporous silica hybrid material (GMS) with gold nanoparticles in the silica walls of GMS.^32,33 The high pore order and the morphology can be controlled very well and the gold nanoparticles are highly dispersed in the silica walls of GMS. The abundant hydroxyl groups on the pore surface enable these materials to interact well with the organic molecules. Thus, encapsulation of an organic molecule enhances its stability and performance in physiological conditions. In addition, since the emission features of the gold nanoparticles are temperature insensitive, it allows the design of a temperature calibrated fluorescent sensor via single wavelength excitation.³⁴

Inspired by the aforementioned concepts, and to address the need for robust and calibrated fluorescent temperature sensing, we describe a novel temperature sensitive dye molecule, 4-(2-hydroxybenzylideneamino)benzoic acid (HBA), which is encapsulated within a gold nanoparticle functionalized mesoporous silica hybrid material (GMS). HBA–GMS shows excellent linear temperature sensitivity from 100 to 298 K. GMS not only improves the chemical stability, but also attenuates the intrinsic toxic effects of HBA. Moreover, HBA–GMS could be used as a ratiometric and calibrated temperature sensor, because the HBA fluorescence intensity can be calibrated using the GMS over a wide temperature range, over which the GMS fluorescence signal is temperature insensitive. Thus, the superstructure design could offer a new approach to construct highly sensitive and selective sensors or cryogenic thermometers.

Results and discussion

In this work, we report a novel gold nanoparticle mesoporous silica hybrid material (GMS) with adsorbed 4-(2-hydroxybenzylideneamino)benzoic acid (HBA), called HBA–GMS, which is synthesized via a simple two step method.³⁷ The first step involves the synthesis of HBA and GMS according to previously reported methods (see ESI S1†). Firstly, under vacuum conditions, the impurities and air trapped inside GMS are completely removed. Then, the GMS that has been vacuum treated and protonated is immersed in an aqueous solution with HBA for 12 hours. Meanwhile, the HBA molecules infiltrate the nanochannels of GMS via capillary action. This nanostructure is denoted as HBA–GMS. In this process, –SiOH groups are protonated to positively charge the –OH. Meanwhile, the negatively charged HBA molecules are attracted to –SiOH₂⁺ and the HBA molecules are adsorbed onto the surface of GMS. Thus, HBA assembles on the positively charged surface of the nanochannels to form anchored HBA molecules as indicated in Scheme 1.

Scheme 1 Schematic representation of HBA adsorbed within the mesoporous GMS and the different electronic structures related to the fluorescence of HBA (cis-keto) and the gold nanoparticles, leading to a dual emission.

Powder X-ray diffraction (XRD) patterns of GMS are shown in Fig. S2a.† GMS is highly ordered, showing two strong diffraction peaks for the 111 and 200 planes. From the XRD patterns, the average crystallite sizes of the gold nanoparticles are calculated with the Scherrer equation using the most intense (111) XRD peak. The calculated crystallite size of the gold nanoparticles is about 2.3 nm, in agreement with the size reported previously (see the ESI S2†).³⁹ Fig. S3† shows the UV-vis absorption spectrum of GMS with a peak at 530 nm, which indicates an average particle diameter of less than 5.0 nm. The nitrogen adsorption–desorption isotherms of GMS are shown in Fig. S2b.† The pore diameter, specific surface area and Barrett–Joyner–Halenda (BJH) pore volume of GMS are 4.7 nm, 539 m² g⁻¹ and 0.52 cm³ g⁻¹, respectively. The isotherms of GMS are type IV, which is typical for a mesoporous structure.⁴⁰ The above results indicate that GMS possesses a typical mesoporous structure, similar to SBA-15, and is suitable to encapsulate small molecules to improve their chemical stability.

Furthermore, Fig. 1a and b show transmission electron microscopy (TEM) images of HBA–GMS and GMS, respectively. As can be seen, the GMS displays highly ordered nanochannels with a channel diameter of ∼4.7 nm and a wall thickness of ∼2.2 nm (Fig. 1b), consistent with the two dimension hexagonal mesostructure of SBA-15, and in agreement with the nitrogen adsorption–desorption isotherm data (Fig. S2b†). The gold nanoparticles are well protected and segregated by the silica walls of GMS, even after calcination at high temperatures. Furthermore, the transmission electron microscopy (TEM) images provide a direct observation of the morphology and distribution of HBA–GMS and GMS, indicating that the nanochannels of HBA–GMS remain uniform with the morphology of GMS. The diameter and shape of the channels of HBA–GMS are as regular as those of GMS. The wall thickness of the HBA–GMS is ∼2 nm, which is similar to the wall thickness of GMS. The results show that the HBA molecules are highly dispersed in the channels of GMS, and HBA does not damage the textural and structure properties of the GMS support (Fig. 1b).

Fig. 1 TEM images of HBA–GMS (a) and GMS (b).

To further confirm the presence of HBA within GMS and its amount, the corresponding Fourier transform infrared (FTIR) spectra and thermogravimetric analyses (TGA) of HBA, GMS and HBA–GMS samples were compared, as shown in Fig. S4.† It can be seen that a series of characteristic bands of HBA, a C–O band (1287 cm⁻¹) and aromatic C=C band (1569 and 1600 cm⁻¹), are present in the spectra of both HBA and HBA–GMS, in agreement with previously reported data for HBA (Fig. S4a†).³⁸ These results also demonstrate that HBA has been successfully adsorbed into the nanochannels of GMS. Moreover, the weight percentage of HBA in HBA–GMS is estimated using thermogravimetric analysis (TGA), as shown in Fig. S4b.† The amount of HBA in the hybrid HBA–GMS is estimated to be ∼28 wt% using TGA.

To further assess the optical properties, the samples were characterized using spectral measurements. Fig. 2a shows the absorption and emission spectra of HBA, GMS and HBA–GMS at 298 K. HBA and HBA–GMS show two absorption peaks at 365 nm and 430 nm at 298 K, respectively. Moreover, the samples were also characterized using fluorescence spectral measurements. HBA and GMS exhibit emission peaks at 564 nm and 416 nm at 298 K, respectively, when excited at 390 nm. HBA–GMS exhibits two emission peaks at around 410–440 nm and a wide band at 510–560 nm at 298 K when excited at 390 nm. The peak at 410–440 nm is therefore attributed to the emission of GMS. The latter broad peak at 510–560 nm is attributed to the emission of HBA. Furthermore, we measured the emission spectra of HBA and HBA–GMS at 8 and 298 K, respectively, as shown in Fig. 2b. In the case of HBA, a narrow emission peak at 564 nm is observed at 298 K and a new peak at around 520 nm is observed with the lower temperature of 8 K, similar to the results reported previously.³⁸ As can be seen for HBA–GMS, the wide peak at 510–560 nm splits into two emission peaks with decreasing temperature. Moreover, the two emission peaks are observed more obviously at temperatures ranging from 298 K to 8 K as shown in Fig. 3a. At the low temperature of 8 K, this wide peak at 510–560 nm splits into two obvious peaks at about 513 nm and 550 nm, which is similar to the optical behavior of HBA (Fig. 2b).³⁸

Fig. 2 (a) Measured absorption spectra of HBA (black hollow star), GMS (black hollow circle) and HBA–GMS (black hollow triangle) and the fluorescence emission spectra of HBA (red hollow star), GMS (red hollow square) and HBA–GMS (red hollow circle) at 298 K. (b) Fluorescence emission spectra of HBA (8 K, solid square; 298 K, hollow star) and HBA–GMS (8 K, hollow triangle; 298 K, hollow circle) at 8 and 298 K. The excitation wavelength is 390 nm.

Fig. 3 (a) Fluorescence emission spectra of HBA–GMS at different temperatures from 8 to 298 K. (b) Temperature dependence of the fluorescence intensity (I₄₁₆, I₅₁₃, I₅₅₀) of HBA–GMS, showing the linear decrease in fluorescence intensity as a function of temperature from 8–298 K. Integrated fluorescence intensity shown as a function of temperature from 8–298 K for HBA–GMS. (c) Fluorescence intensity ratios I_HBA/I_GMS (I₅₁₃/I₄₁₆, I₅₅₀/I₄₁₆ and I_410–440/I_510–560) of HBA–GMS as a function of temperature. (d) Fluorescence intensity ratios I_HBA/I_GMS of HBA–GMS as a function of temperature from 100–298 K.

To evaluate the temperature-induced fluorescence intensity change of HBA–GMS, the fluorescence spectra of HBA–GMS were measured at different temperatures. Fig. 3a shows the fluorescence spectra of HBA–GMS at temperatures ranging from 8 to 298 K when excited at 390 nm. As can be seen, a gradual decrease in the fluorescence intensity with increasing temperature can be observed. The emission peak shifts towards longer wavelengths from 510 to 560 nm with increased temperature. HBA–GMS shows a strong temperature-dependent fluorescence with a very high quantum yield over a wide temperature range, from 8 to 298 K, as shown in Table S1.† The total quantum yields of the fluorescence are 0.76 and 0.2 at 8 and 298 K, respectively.

Furthermore, we have plotted the change in the fluorescence intensity of the peaks at wavelengths 416, 513 and 550 nm and the integrated PL intensity as a function of temperature from 8 to 298 K, which could be used for accurate temperature sensing (Fig. 3b). For HBA–GMS, as can be seen, the fluorescence intensity and the integrated fluorescence intensity change linearly over the represented temperature range, between 8 and 298 K. The results show that there is a good linear relationship between the PL intensity and temperature. To avoid disturbance of the HBA optical signals by the surroundings, we have plotted the ratio of the HBA emission to the gold nanoparticle emission to calibrate the fluorescence as a function of temperature. Fig. 3d shows the fluorescence intensity ratio (I₅₁₃/I₄₁₆, I₅₅₀/I₄₁₆) and the integrated intensity (I_510–560/I_410–440) as a function of temperature. As can be seen, a good linear correlation between the fluorescence intensity and integrated intensity ratio vs. temperature from 100 to 298 K is observed, as shown in Fig. 3d. The ratio of I_HBA/I_GMS (I₅₁₃/I₄₁₆, I₅₅₀/I₄₁₆ and I_510–560/I_410–440) against temperatures from 100 to 298 K reveals a good linear relationship with correlation coefficients of 0.97–0.99. The absolute temperature can therefore be linearly correlated to an experimental parameter Δ using eqn 1

Δ = 22.467 − 0.0499T (1)

where Δ = I_510–560/I_410–440 is the emission intensity ratio at different temperatures between 100 and 300 K. This suggests that HBA–GMS is an excellent luminescent thermometer in this temperature range. As shown in Fig. 3d, the sensitivity of HBA–GMS is significantly enhanced and is higher than that of a recently reported metal–organic framework.^11,13 Such significantly enhanced sensitivity is really remarkable, and will allow us to highly sensitize the temperature changes.

Material biocompatibility is a key factor for biological applications. Thus, to fully evaluate the possible toxicity of HBA and HBA–GMS, the cell viability of MEFs (mouse embryo fibroblasts) treated with different concentrations of HBA and HBA–GMS for 24 hours was evaluated, respectively. The cell viability of MEFs reduce from 82% to 38% when the concentration of HBA increases from 25 to 400 μg ml⁻¹, as shown in Fig. 4. The cell viability of MEFs is maintained at around 100% with concentrations of HBA–GMS from 25 to 400 μg ml⁻¹ (Fig. 4). It is concluded that HBA–GMS at concentrations up to 100 μg ml⁻¹ does not show significant toxicity with the current experimental conditions. This may be attributed to the fact that GMS is able to insulate and stabilize HBA molecules and, therefore, inhibit the possible interactions between the HBA molecules and the surfaces of the cells.

Fig. 4 Cell viability of MEF cells in the presence of HBA (orange filled) and HBA–GMS (gray filled), evaluated using the MTT assay.

To investigate the toxicity of HBA–GMS in skin cancer cells, a mixture of B16 cells (1.0 × 10⁴ cells per well) were treated with HBA–GMS (100 μg ml⁻¹) for 5 hours (Fig. 5). Meanwhile, B16 cells were also incubated with HBA for 5 hours as a control sample (Fig. 5a and b). B16 cells grown in a medium were treated with both PI (10 μg ml⁻¹), for staining the nuclei of dead cells, and Hoechst 33342 (5 μg ml⁻¹), for staining the nuclei of living cells, and each condition was analyzed using Image Pro. The cells that were treated with HBA showed more cell death (Fig. 5a and b), whereas the cells incubated with HBA–GMS were unaffected (Fig. 5c and d). Although the initial cell numbers were at the same level in all experiments, the staining procedure with trypan blue includes several washing steps. Thus, most of the cells incubated with HBA were discarded, which resulted in a cell number difference compared with HBA–GMS. Meanwhile, this washing process of the staining also prevented influence of the sample fluorescence signal. In conclusion, HBA–GMS exhibited an almost absent inhibitory effect on the viability and growth of the cell suspension cultures in comparison with HBA without GMS. Since the HBA–GMS binds readily to the surface of cells, the reduction of the HBA toxicity is owed to the protection from the mesoporous silica walls, avoiding direct contact with the cells. This result demonstrates that HBA–GMS exhibits low toxicity and provides a fluorescent thermometer tool for nanobiotechnology, biomedicine, and animal fields.

Fig. 5 Fluorescence images of B16 cells treated with HBA (a) and (b) and HBA–GMS (c) and (d). Hoechst 33342 was used to visualize the nuclei (blue (a) and (c)) and PI to illustrate the apoptotic cells (red (b) and (d)) (the amount of dead cells is expressed as a percentage of the PI-positive cells).

Conclusions

In summary, a new temperature responsive salicylideneaniline (HBA), which is encapsulated within a mesoporous silica functionalized with gold nanoparticles (GMS), is demonstrated. HBA–GMS acts as a calibrated fluorescent sensor, showing a strong and linear temperature-induced fluorescence intensity in the temperature range of 100 to 298 K. Moreover, GMS not only improves the chemical stability and biocompatibility of HBA, but also eliminates its intrinsic toxicity effectively. GMS also improves the light harvesting and calibrates the HBA fluorescence intensity over a wide temperature range because of the stable fluorescence signal of the gold nanoparticles. Thus, HBA–GMS as a fluorescent thermometer can be used for colorimetric imaging. These properties make HBA–GMS a sensitive fluorescent colorimetric thermometer which could be used in many areas, such as temperature distribution mapping.

Experimental

Preparation of HBA–GMS composites

4-(2-Hydroxybenzylideneamino)benzoic acid (HBA) and GMS were synthesized using a previously reported procedure (see ESI S1† for detailed experimental procedures).^35,36 For a typical preparation, 2.5 mg of HBA was dissolved in 50 ml of ethanol first, followed by adding 0.5 g of GMS into the solution and mixing for 12 hours at room temperature. The suspension was centrifuged and the transparent aqueous solution was decanted. The solid was carefully washed with ethanol to remove HBA and dried in a vacuum oven at 333 K. The solid powder was obtained and denoted as HBA–GMS.

In vitro cytotoxicity assay (MTT assay)

The viability and proliferation of cells in the presence of HBA and HBA–GMS were evaluated via a MTT assay using a previously reported procedure (see ESI S1† for the detailed experimental procedures).⁴¹ A Hoechst 33342/PI stain was used for the cell death assay, Hoechst 33342 was employed for staining the cell nuclei and the red signal of PI indicated dead cells. Cells treated with the tested materials were stained with PI (0.5 mg ml⁻¹) and Hoechst 33342 (5 μg ml⁻¹) for 20 min and washed with PBS three times. All measurements in the experiments are made at room temperature.

Characterizations

Nuclear magnetic resonance (NMR) spectra were determined using a Bruker AV300 NMR spectrometer (resonance frequency of 300 MHz for 1 H) operated in Fourier transform mode. CDCl₃ was used as the solvent. The XRD patterns were recorded on a Philips X′ Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The field emission scanning electron microscopy (FESEM) images were performed using a FEI Sirion-200 scanning electron microscope. The transmission electron microscopy (TEM) images were taken on a JEOL-2010 TEM with an acceleration voltage of 200 kV. The porous textures of the samples were analyzed using nitrogen adsorption/desorption isotherms at 77 K. The nitrogen adsorption/desorption isotherms were determined using a Micromeritics ASAP 2000 system. The FTIR spectra were measured on a NICOLET FT-IR spectrometer, using pressed KBr tablets. Thermogravimetric analysis (TGA) of the samples was measured on a Shimadzu TA-50 thermal analyzer at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C in air. The UV-Vis spectra were carried out on a Solid Spec-3700 spectrophotometer at room temperature. Fluorescence measurements were determined using a FluoRoLOG-3-TAU (Jobin Yvon, France) fluorescence spectrometer, which included a xenon lamp equipped with a grating monochromator. Low temperatures were achieved using CCS-355 (Janis, America) low temperature equipment.

Acknowledgements

This work is financially supported by the Agency for Science, Technology and Research (A*STAR) and Ministry of National Development (MND) Singapore (grant #: SERC 1321760014). We would like to acknowledge the partial support from the National Natural Science Foundation of China (No. 51302282).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and additional experimental data. See DOI: 10.1039/c5ra09626j

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