Fast and Reversible Humidity-Responsive Luminescent Thin Films

Dec 18, 2015 - Katalin Szendrei , Alberto Jiménez-Solano , Gabriel Lozano , Bettina V. Lotsch , Hernán Míguez. Advanced Optical Materials 2017 5 (23),...
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Fast and Reversible Humidity-Responsive Luminescent Thin Films Rui Gao,‡ Ding Cao,‡ Yan Guan,§ and Dongpeng Yan*,†,‡ †

Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China ‡ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing 100029, P.R. China § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: Highly sensitive stimuli-responsive fluorescent films are playing an increasingly important role in the development of smart sensors and erasable optical devices. However, systems involving humidity-responsive fluorescence (HRF) are still very limited compared to those responsive to other common environmental stimuli (e.g., light, heat, pressure, or pH). Herein, by incorporating the 4-[4-(dimethylamino)styryl]pyridine chromophore into a polyvinylpyrrolidone host, we have developed new flexible self-supporting nanofiber films that exhibit fast and obvious HRF. The reversible transformation between two fluorescence states can be easily observed and recycled at least 200 times. Fluorescence microscopy images provided in situ evidence of changes in both fluorescence and morphology. This work therefore offers an alternative to conventional humidity sensors based on changes in color and electrical properties. Furthermore, we anticipate that these HRF films can also be employed as optical antiforgery materials.

1. INTRODUCTION Molecular materials with stimuli-responsive functionalities are expected to have wide applications in future intelligent switches and sensoring devices.1−4 Recently, stimuli-responsive fluorescent materials have attracted great attention, because of their sensitive signals and easy identification and imaging.5−8 Although several types of fluorescence switching systems sensitive to external stimulisuch as thermo-,9 mechano-,10 pH-,11 and light-induced12 chromic luminescent materials have been developed, several challenges remain unresolved. For example, application of such materials in readable/erasable smart systems requires high visualization contrast ratios, fast response times, and stable reversibility.13,14 Moreover, if smart luminescent materials are to be incorporated in practical devices, it is highly desirable for ordered thin films of the material to be fabricated, rather than using solution or powdered forms.15,16 Unfortunately, such films have rarely been reported.17,18 Humidity is one of the most common naturally occurring external stimuli, and thus, humidity-responsive fluorescence (HRF) materials can be used for the monitoring of humidity changes in environmental control, agricultural production, food storage, and many chemical processes (e.g., in the pharmaceutical industry).19−22 Compared with the well-developed fluorescent systems responding to other external stimuli mentioned above, the study of HRF materials remains in its infancy, and examples are still rather rare.23−28 In addition, current humidity sensor systems are generally based on changes in semiconducting/electrical properties (such as ceramic-based sensors29−31) or colors (such as photonic crystals32,33). Ceramic-based humidity sensors have been reported to have good responsive ability only over 30% relative humidity (RH) at room temperature.20,21 Also, the poor stability of such © 2015 American Chemical Society

materials is usually difficult to overcome. For humidity sensors combining the intrinsic sensitivity of hydrogels and the structural color of photonic crystals, the materials have good humidity-responsive visualization, but the response time is relative long (usually at least 5 min).33 Therefore, there is an increasing demand for a wide-range sensitivity and much faster humidity response for practical applications. In this work, we have developed new types of reversible HRF thin films by introducing 4-[4-(dimethylamino)styryl]pyridine (DSP) (Figure 1A) into polyvinylpyrrolidone (PVP) nanofibers using an electrospinning process (Figure 1B). DSP was chosen because it contains a pyridine group that can potentially form hydrogen-bonding interactions with water molecules (Figure 1D), which can influence the electronic structure of DSP. Different extents of humidity can also change the aggregation states of the hydrophobic DSP, thus allowing tailoring of its fluorescence properties. The electrospinning method is a versatile wet technique for the fabrication of continuous micro-/nanosized fibers, which can be further directly assembled into thin films.34−37 The porous environment generated in the electrospinning process should also facilitate the adsorption/desorption of water molecules. The choice of PVP as the electrospun matrix is based on the expectation that interactions between the PVP host and DSP guest molecules (Figure 1C) will favor the formation of a locally ordered orientation and high dispersion of DSP molecules within the fibers, leading to uniform and systematic changes in fluorescence when exposed to different external humidities. Received: Revised: Accepted: Published: 125

September 11, 2015 November 9, 2015 December 18, 2015 December 18, 2015 DOI: 10.1021/acs.iecr.5b03389 Ind. Eng. Chem. Res. 2016, 55, 125−132

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Industrial & Engineering Chemistry Research

Figure 1. Schematic illustration of (A) the incorporation of DSP molecules into PVP fibers as (C, D) humidity-sensitive fluorescent materials using (B) an electrospinning method.

The as-prepared DSP@PVP fiber films present fast and obvious HRF behaviors at RHs varying in the range from 16% to 95%. The luminescent properties of the fiber films can be recovered quickly, and the process can be recycled hundreds of times. A morphological study indicated that there was a softening of the fiber surface under high RHs that is related to solvation effects on the DSP molecules. Therefore, this work provides a feasible route for developing flexible humidity-controlled fluorescent thin films with potential applications in luminescent switches and new smart antiforgery materials.

Additionally, the relatively high photoluminescent quantum yield values for DSP@PVP thin films (20.35%, 20.39%, and 20.00% for x% = 0.5, 1.0, and 1.5 wt %, respectively) ensure their easy observation and fluorescence imaging. To probe the morphological features of DSP-based selfsupporting films (Figure 2B), DSP@PVP systems were further studied by scanning electron microscopy (SEM). It can be observed (Figure 2B) that the films were constructed from fibers with a width of ca. 200 nm. Each individual nanofiber had an extremely high length and aspect ratio and that the nanofiber surface was continuous and homogeneous. The surface structure of the fibers was also monitored by atomic force microscopy (AFM), which showed that the individual fiber surface was continuous and uniform (Figure 2B, right) and that the fibers had thicknesses of 230−300 nm. Considering the nonlinear optical properties of DSP molecules,39 the upconversion emission of the DSP@PVP(x%) films was monitored using excitation by an 800-nm laser at different excitation powers (Figure 2C). The up-conversion emissive positions of the films were located at 451, 462, and 467 nm for x% = 0.5, 1.0, and 1.5 wt %, respectively, which are consistent with those observed upon UV excitation. Moreover, the logarithm of the intensity exhibited a good linear relationship with the logarithm of the incident energy, with a slope close to 2, suggesting that the emissive processes involve a two-photon emission mechanism. By in situ monitoring (Figure 3A) of the fiber films in atmospheres with different RH values, we observed that all of the thin films underwent changes in luminescence to different extents when the RH was increased from 16% to 95%. [The maximum photoemission wavelength (λmax em ) moved from 451 to 475 nm for x% = 0.5%, from 462 to 489 nm for x% = 1.0%, and from 467 to 489 nm for x% = 1.5% (Figure 3).] The fluorescence reached a stable value within only 2 s: Such highspeed humidity-sensitive fluorescence changes have not previously been reported. For example, it was reported that salicylic acid doped poly(vinyl alcohol) film can be used as an optical humidity sensor with a response time of about 2 min and that there are changes in only the intensity rather than the

2. RESULTS AND DISCUSSION By means of the electrospinning method, DSP molecules were incorporated into a PVP matrix at different mass concentrations (typically, x% = 0.5, 1.0, and 1.5 wt %) to form flexible selfsupporting DSP@PVP(x%) films. XRD profiles of the DSP@ PVP films showed no diffraction peaks from either DSP or PVP, suggesting the formation of an amorphous composite material [Figure S1 of the Supporting Information (SI)]. When excited with a UV light at 365 nm, the resulting films exhibited tunable fluorescence, with a red shift of the emission being observed with increasing concentration of DSP, from 451 nm for 0.5 wt % to 467 nm for 1.5 wt % (Figure 2A), indicating that increasing the DSP concentration leads to the formation of J-type molecular aggregates and/or excimers of DSP molecules within the polymer films.38 Such a wavelength-shift effect is related to the fact that DSP exhibits single-molecule luminescence at relatively low concentrations and that the emissive and/or aggregation excimer appears when the content of DSP increases to a certain concentration, resulting in a red shift of the emission. The fluorescent emissions at different concentrations are uniform and homogeneous, suggesting that the DSP chromophores are highly dispersed in the PVP films (Figure 2B). FT-IR spectra (Figure S2 of the SI) showed that the characteristic peaks of DSP@PVP thin films are mostly the same as those of PVP, further indicating that the DSP chromophores are highly dispersed in the PVP films. 126

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3D, inset). In addition, the maximum absorption band of the film exhibited a red shift of 24 nm from 342 to 366 nm when the RH was increased from 16% to 95% (Figure S3C, SI), which is consistent with the change in fluorescence. The change in the wavelength range (22-nm shift) from 16% to 95% RH is more sensitive than that of the previous dapoxyl sulfonic acid incorporated hydrogel system (15-nm shift),25 and the HRF in this work is also comparable the response of a temperatureresponsive fluorescence sensor (29 nm). 14 To obtain information about the excited state of the fiber film before and after the change in external humidity, we measured the fluorescence lifetimes of the films under low and high RH in an in situ manner. We observed that the fluorescence decay became faster under high RH (SI, Figure S4) and that the corresponding fluorescence lifetime was reduced from 1.46 ns (for 16% RH) to 1.11 ns (for 95% RH) for DSP@PVP(1.5%). To better understand the humidity-sensitive spectral change of the DSP-based film, time-dependent density functional theory (TDDFT) calculations were performed on two idealized DSP and DSP·H2O models. As shown in Figure S3A,B (SI), the simulated UV−vis spectra are close the experimental spectra, with the absorption bands of DSP exhibiting a calculated red shift of 13 nm after interacting with water molecules, as a result of the increase in the extent of π-conjugation of the DSP units. Similar red shifts of the absorption spectra (Figure S5, SI) under high-RH conditions also occurred for DSP@PVP(0.5%) and DSP@PVP(1.0%). FT-IR spectra showed that the characteristic vibration bands of the carbanyl group and the CN group in DSP@PVP films systematically moved to lower wavenumbers (by approximately 9−11 cm−1) when the film was transferred from 16% to 95% RH (Figure S6, SI). These observations indicate that the formation of hydrogen bonds between DSP and water molecules reduced the frequency of vibration of the DSP@PVP film. The changes in the isolated nanofibers were further observed under a fluorescence microscope, with the fluorescence color of the individual nanofibers showing an obvious change as the RH was increased from 16% to 95% (Figure 4). For DSP@ PVP(1.5%), the corresponding emission color changed from blue [CIE 1931 color coordinates (0.161, 0.231); Figure 3C, right] to cyan [CIE 1931 color coordinates (0.117, 0.381)]. In addition, a softening of the nanofibers was also detected (denoted by the white circles in Figure 4C). Similar changes in fluorescence color and morphology were also observed for DSP@PVP(0.5%) and DSP@PVP(1.0%), as shown in panels A and B, respectively, of Figure 4. To the best of our knowledge, such in situ dynamic observation of the humidity-controlled fluorescence change of nanofibers has not been previously reported. Moreover, when exposed to even higher RH conditions (98%) for more than 10 min, it was found that a single nanofiber tended to dissolve, giving small droplets, as observed in the microscope (Figure 4D). This suggests that the solvation effect of water molecules on DSP@PVP might play a key role in the changes in luminescence, because the hydrophobic DSP and PVP both tend to form locally aggregated states under high humidity. Good reversibility and reproducibility are highly important if the films are to have practical applications as HRF materials. When the DSP@PVP(1.5%) film was transferred from high RH to low RH and kept for 20 s, the fluorescence recovered its original color and spectrum completely (Figure 5A). The reversible changes in photoemission can be repeated more than 200 times (Figure 5B), and the fluorescence lifetime (SI, Figure

Figure 2. (A) Fluorescence spectra of DSP@PVP nanofiber films at different concentrations. (B) Photographs of macro-sized fiber films (left), corresponding SEM images (middle), and AFM images of selected single fibers (right). (C) Fluorescence spectra of fiber films excited by an 800-nm laser at different pump powers.

wavelength of fluorescence when exposed to environments of different RH values.26 In another work, dapoxyl sulfonic acid was incorporated into a hydrogel that exhibited a 15-nm wavelength shift and a response times of ca. 5 min.25 In this work, taking the DSP@PVP(1.5%) film as an example, as the RH was increased from 16% to 95%, the uniform change in fluorescence color, the wavelength of which moved from 467 to 489 nm (Figure 3D), was easily visible to the naked eye (Figure 127

DOI: 10.1021/acs.iecr.5b03389 Ind. Eng. Chem. Res. 2016, 55, 125−132

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Figure 3. (A) Schematic illustration of in situ fluorescence detection under different RH conditions. (B−D) Fluorescence spectra of DSP@PVP fiber films at different RHs: (B) DSP@PVP(0.5%), (C) DSP@PVP(1.0%), and (D) DSP@PVP(1.5%).

Figure 4. (A−C) Photographs of fluorescence microscope images and changes in the color coordinates of films at different RHs: (A) DSP@ PVP(0.5%), (B) DSP@PVP(1.0%), (C) DSP@PVP(1.5%). (D) Change in morphology of a single nanofiber at a RH of 98%.

observed for the DSP films with other concentrations (Figure 6). To the best of our knowledge, such a long cycling time and repeatable performance have seldom been reported. Moreover,

S4C, inset) also showed a corresponding repeating change, showing the potential for application of the materials in practical humidity sensors. Similar reversible changes were also 128

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we compared the sensitivity and response time of the DSP@ PVP film with a commercial moisture meter that utilizes the changes in electrical resistance with varying RH. The commercial moisture meter required ca. 2.8 and 8.3 min to reach a stable reading when the RH was varied from 16% to 95% and from 95% to 16%, respectively. However, the fluorescence of the film required only 2 and 20 s, respectively, to reach a stable value under the same conditions. This confirms the advantages of the fluorescence response for detecting rapid changes in external RH, particularly under conditions of extreme humidity. As another example, the fluorescence of the films changed immediately when it was breathed upon (Figure 7A,B), and the luminescence color was recovered within 20 s, indicating the facile and easy operation of the humidity-sensitive DSP@PVP film. This erasable luminescent signal offers potential applications in fluorescence switching (such as in fluorescent antiforgery materials). Upon selective-zone humidity treatment of the film using a shadow mask for 2 s, luminescent patterned films were easily obtained, in which the areas exposed to humidity exhibit an obvious redshifted emission (Figure 7C). Thus, this strategy offers a facile way to develop new types of luminescent patterns and readable array films with any desired shape. To detect the HRF film at high temperature, the fluorescence spectra of DSP@PVP(1.5%) were also measured at 50 °C, and it was found that the film exhibited no shift in the emission wavelength at different temperatures (Figure S7A, SI). Also, it was observed that the thin films underwent changes in luminescence to different extents as the RH was increased from 7% to 95% at 50 °C

Figure 5. (A) Fluorescence spectra of the DSP@PVP(1.5%) fiber film at different RHs (16% and 95%) over one cycle and (B) reversible fluorescence responses (λmax em ) over 200 consecutive cycles.

Figure 6. (A,C) Fluorescence spectra of fiber films at alternating relative humidities (16% and 95%) over one cycle and ( B,D) reversible fluorescence responses (λmax em ) over 15 consecutive cycles for (A,B) DSP@PVP(0.5%) and (C,D) DSP@PVP(1.0%). 129

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Figure 7. (A) Fluorescence spectra of the DSP@PVP(1.5%) fiber film before and after exposure to human breath and (B) corresponding fluorescence photographs. (C) Fluorescence imaging by selective-zone humidity treatment of the DSP@PVP(1.5%) film.

Preparation of Electrospinning Solutions. Typically, 5.3 g of PVP was first dispersed into 20 mL of methanol for 0.2 h to obtain a PVP solution. Then, 0.0265, 0.053, or 0.08 g of DSP was added to the PVP solution under vigorous stirring. The resulting clear homogeneous solution was used for electrospinning and the fabrication of DSP@PVP films (with mass concentrations of 0.5, 1.0, or 1.5 wt %, respectively). Electrospinning and Casting Films. A buret with an inserted Cu rod connected to a high-voltage supply was filled with the DSP@PVP aqueous solution. An Al sheet connected to the ground was used as the receiver. The distance between the buret tip and the receiver was fixed at 18 cm, and the highvoltage supply was fixed at 25 kV. The spinning rate was controlled at about 5 mL h−1 by adjusting the angle of inclination of the buret. Pieces of Al sheet about 40 cm × 40 cm were placed on the Al sheet for collecting samples. Then, 0.5 mL of the composite solution was dropped onto a clean Al sheet and left to dry under ambient conditions to form thin films. Humidity Testing of DSP@PVP Fiber Films. Environments with various relative humidities were created with the aid of saturated salt solutions. Salts were chosen to produce relative humidity (RH) values as reported before (K2CO3·2H2O, 43%; Mg(NO3)·6H2O, 54%; KI, 70%; KCl, 85%; KNO3, 95%; K2SO4, 98%). Saturated solutions of each salt were prepared at room temperature in wide-mouth bottles for 24 h. The fluorescence spectra of DSP@PVP fiber films were recorded by in situ monitoring in air of different RH values. First, we immobilized the fiber film in a sealed quartz plate and put the quartz plate in the detection location of the fluorescence spectrometer. We detected the fluorescence spectra of the film while injecting different RH moisture inside the quartz plate. Sample Characterization. UV−vis absorption spectra were collected in the range from 200 to 600 nm on a Shimadzu U-3000 spectrophotometer, with a slit width of 1.0 nm. The fluorescence spectra were recorded on RF-5301PC

(Figure S7B, SI), which are close to the behaviors at room temperature, suggesting that the HRF film can be applied in a wide temperature range in daily life.

3. CONCLUSIONS In summary, we have confirmed that luminescence can achieve a fast and reversible humidity response which offers an alternative to conventional humidity detection via well-known mechanisms (such as changes in electrical properties or color). The fluorescent film materials can be easily fabricated by incorporation of a DSP chromophore into a PVP matrix using an electrospinning process. At different RH values, the selfsupporting nanofiber films exhibited immediate and obvious changes in luminescence (such as emissive color and fluorescence lifetime). Confocal fluorescence microscopy scanning was used to detect the changes in fluorescence and morphology in situ for an individual nanofiber. The short response time and stably reversible humidity-controlled fluorescence promise potential future applications in humidity detection devices. In addition, the easy recognition of the fluorescence transformation under low/high RH should also allow the films to be employed as optical antiforgery materials. To the best of our knowledge, this system might involve the first example of nanofiber films having reversible humiditysensitive luminescence. It is to be expected that the incorporation of other stimuli-sensitive fluorescent molecules in flexible polymers can be utilized to fabricate a range of new smart luminescent materials. 4. EXPERIMENTAL SECTION Reagents and Materials. 4-[4-(Dimethylamino)styryl]pyridine (DSP) was obtained from Sigma Chemical Co. Ltd. and used without further purification. Polyvinylpyrrolidone (PVP) (Mn = 1333, 99% hydrolyzed) and methanol were purchased from Beijing Chemical Co. Ltd. and used without further purification. 130

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Industrial & Engineering Chemistry Research fluorospectrophotometer with an excitation wavelength of 365 nm, with the excitation and emission slits both set to 3.0 nm. The fluorescence decays were measured using a LifeSpec picosecond spectrometer with a 372-nm laser exciting the films, and the fluorescence lifetimes were calculated with F900 Edinburgh Instruments software. The morphologies of the thin films were investigated using a scanning electron microscope (SEM Zeiss Supra 55) equipped with an energy-dispersive Xray (EDX) attachment, and the accelerating voltage applied was 20 kV. The surface roughness and thickness data were obtained using a Bruker Multimode 8 atomic force microscope. Photoluminescence quantum yield (PLQY) and CIE 1931 color coordinates were measured using an HORIBA JobinYvon FluoroMax-4 spectrofluorimeter equipped with an F-3018 integrating sphere. Two-photon-excited fluorescence of the samples was excited by an 800-nm laser on a Tsunami-SpitfireOPA-800C ultrafast optical parameter amplifier (Spectra Physics). The fluorescence images were obtained on an Olympus U-RFLT50 fluorescence microscope. Computational Methods. The structural optimization and the UV−vis spectra of DSP and DSP·H2O were calculated by the time-dependent density functional theoretical (TDDFT) method at the B3LYP/6-31G(d,p) level using the Gaussian 03 suite of programs.40

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03389. XRD profiles of fiber films (Figure S1), FT-IR spectra of fiber films and pristine DSP and PVP samples (Figure S2), simulated and experimental UV−vis spectra of the DSP@PVP fiber film (Figure S3), typical fluorescence decay curves of fiber films at RHs of 16% and 95% (Figure S4), UV−vis spectra of the films at 16% and 95% RH (Figure S5), FT-IR spectra of the films at different RH values (Figure S6), and fluorescence spectra of the fiber film at different temperatures and at different RHs at 50 °C (Figure S7) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-64425385. E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (Grant no. 2014CB932103), the National Natural Science Foundation of China (NSFC), Beijing Municipal Natural Science Foundation (Grant no. 2152016), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

(1) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Rapid and Reversible Shape Changes of Molecular Crystals on Photoirradiation. Nature 2007, 446, 778. (2) Fleischmann, E. K.; Zentel, R. Liquid-Crystalline Ordering as a Concept in Materials Science: from Semiconductors to StimuliResponsive Devices. Angew. Chem., Int. Ed. 2013, 52, 8810. 131

DOI: 10.1021/acs.iecr.5b03389 Ind. Eng. Chem. Res. 2016, 55, 125−132

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DOI: 10.1021/acs.iecr.5b03389 Ind. Eng. Chem. Res. 2016, 55, 125−132