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Ultra-Bright Fluorescent Silica Nanoparticle Embedded with Conjugated Oligomers and Their Application for Latent Fingerprint Detection Shijie Zhang, Ronghua Liu, Qianling Cui, Yu Yang, Qian Cao, Wenqiang Xu, and Lidong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15612 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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ACS Applied Materials & Interfaces

Ultra-Bright Fluorescent Silica Nanoparticle Embedded with Conjugated Oligomers and Their Application for Latent Fingerprint Detection Shijie Zhang,# Ronghua Liu,# Qianling Cui,* Yu Yang, Qian Cao, Wenqiang Xu, and Lidong Li*

#

These authors contributed equally to this study.

State Key Lab for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Email: [email protected]; [email protected] Keywords:

fluorescence,

conjugated

oligomer,

silica

fingerprints, high quantum yield

1

ACS Paragon Plus Environment

nanoparticles,

latent

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Fluorescent micro- and nano-sized particles have a broad range of applications in biology, medicine, and engineering. For these uses the materials should have high emission efficiency and good photostability. However, many organic fluorophores suffer from aggregation-induced quenching effects and photobleaching. Here, we used a simple method based on covalently blending a fluorescent conjugated oligomer with silica nanoparticles to achieve emission quantum yields as high as 97%. The resulting system also showed excellent stability under continuous light illumination, a range of pH values and temperatures, and in common solvents. This fluorescent material showed outstanding properties including highly efficient blue emission, low cost, low toxicity, and easy synthesis. Furthermore, its effectiveness for latent fingerprint detection was demonstrated as a proof of concept on various substrates. The obtained emissive fingerprint powder gave good optical/fluorescent images with high contrast between the ridges and spaces and resolution. Introduction Conjugated polymers (CPs) possess strong light-harvesting abilities and many desirable optical properties, which have attracted particular interest for applications as fluorescent materials in chemical and biological sensing, cell imaging, and photodynamic therapy.1-8 In recent years, conjugated oligomers have drawn attention, as simpler analogues of CPs with the same π-conjugated backbone and a shorter degree of polymerization, giving a well-defined molecular weight.9-12 Compared with conjugated polymers, conjugated oligomers are easier to synthesize, purify, and manipulate. Furthermore, certain unique fluorescence properties, arising from the 2


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discrete structure of oligomers, show promise for applications in sensing, imaging, antibacterial agents, drug/gene delivery, and controlled release.13-17 Nevertheless, both conjugated polymers and oligomers suffer from aggregation caused quenching (ACQ) when transferred from solution to a nanoparticle form or a solid film. This effect is attributed to aggregation-induced depopulation of excitons.18,19 The ACQ problem greatly reduces the emission efficiency of such materials and restricts their applications in the solid state. To reduce ACQ, a popular and effective strategy is incorporating structural part with aggregation-induced emission (AIE) effect proposed by Tang et.al in 2001.20-23 As an alternative way, organic matrices are widely used to disperse the fluorophore, particularly suitable for CPs.24 Typically used amphiphilic polymer matrices include poly(styrene-g-ethylene oxide) (PS-PEG-COOH), poly(styrene-co-maleic anhydride) (PSMA), and poly(DL-lactide-co-glycolide) (PLGA).16,25-27 Recently, a conjugated oligomer was introduced into a sucrose matrix and showed a markedly enhanced quantum yield (43%) compared with that of the neat fluorophore.28 Other potential matrices for dispersing fluorophores include those based on silica nanoparticles (NPs), which can improve physicochemical stability by isolating the dye from external perturbation.29,30 Silica NPs are widely applied in materials and biomedical fields because of their controllable size, optical transparency, ease of modification, facile synthesis, and low toxicity. Covalent immobilization of a fluorophore in a silica matrix has been shown to be a promising strategy for generating fluorescent materials with high emission efficiency.30-38 3



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Fingerprints are widely used biometric information for identity recognition and criminal investigations, because of the unique patterns of an individual’s fingerprint ridges.39 Among methods used to develop latent fingerprints, fluorescence based techniques give high sensitivity and spatial resolution.40-44 Among these examples, one

approach

for

imaging

latent

fingerprints

based

on

electrogenerated

chemiluminescence on electrode surface is worthy mentioned, as it had effectively avoid the ACQ problem.45,46 Recently, there have been several reports on the use of CPs and NPs as fluorescent agents, which have shown good performance for fingerprint development.47-51 However, in these cases a surfactant was required to assistant adsorption of the CPs or NPs to the fingerprint ridges, or another post-treatment was required to realize visualization of the fingerprints. The development of more efficient and cheaper nontoxic fluorescent materials for latent fingerprints detection is highly desirable. In this study, we report silica NPs, with a silane-modified conjugated oligomer incorporated into a silica matrix by a reverse micelle method, which yielded particles with ultra-bright blue-emission and quantum yields as high as 97%. These fluorescent NPs exhibited stable optical properties and no photodegradation was observed under continuous irradiation. Furthermore, the NPs could be dispersed in many common solvents and showed little fluctuation across a wide range of pH values and temperatures. The enhanced optical properties could be attributed to protection of the conjugated oligomer, which was covalently bound to the silica matrix. Thus, a simple and scalable synthesis method was used to generate ultrabright and photostable 4


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fluorescent powders. The resulting hybrid NPs could be used as a dusting material for effective latent fingerprints detection. Experimental Section Materials.

Tetrakis-(triphenylphosphine)palladium(0)

(Pd(PPh3)4),

platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (Pt(dvs)) solution in xylene, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) (Mn = 15834, Mw = 58200) and Triton X-100 were purchased from Sigma-Aldrich and used as received. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(6′-bromohexyl)fluorene (compound 2) was synthesized according to a reported procedure.52 2-Bromofluorene, allyl bromide, tetrabutylammonium bromide (TBAB), trimethylamine (Me3N) solution in ethanol (30 wt%), tetraethyl orthosilicate (TEOS), trimethoxysilane (HSi(OMe)3), ammonium hydroxide solution (NH3·H2O) were purchased from J&K Chemical and used without further purification. All solvents were purchased from Beijing Chemical Works and used as received unless otherwise stated. Tetrahydrofuran (THF) was distilled from Na/diphenylketone. Ultrapure Millipore water (18.6 MΩ·cm) was used throughout the experiments. Synthesis of Compound 1. 2-Bromofluorene (5.07 g, 20.6 mmol), TBAB (1.31 g, 4.1 mmol) and allyl bromide (25 g, 206 mmol) were mixed thoroughly with potassium hydroxide (KOH) solution (50 wt%, 41.3 mL). The mixture was stirred for 15 min at 75 °C under reflux conditions. After cooling to room temperature, the reaction was extracted with dichloromethane (CH2Cl2), and washed with water. The organic layer was dried over anhydrous sodium sulfate and the solvent was removed. 5



The crude product was purified by column chromatography on silica gel with hexane as an eluent to afford 1 (5.58 g, 83%). 1H NMR (400 MHz, CDCl3), δ 7.66 (dd, 1H), 7.55 (d, 2H), 7.49–7.43 (m, 1H), 7.39 (dt, 1H), 7.37–7.27 (m, 2H), 5.24 (ddt, 2H), 4.81 (dd, 4H), 2.69 (d, 4H). MS (EI): calcd: 325.25, found: 326.0. Synthesis of Compound 3. Compound 1 (2.33 g, 7.16 mmol) and compound 2 (2.14 g, 2.87 mmol) were dissolved in 48 mL of toluene, and then 12 mL of 2.0 M potassium carbonate (K2CO3) solution was added. After the solution was degassed with argon for 30 min, the catalyst Pd(PPh3)4 was added. The reaction mixture was stirred at 85 °C for 48 h under an argon atmosphere. The reaction mixture was extracted with CH2Cl2, and washed with brine. The organic layer was then dried over anhydrous sodium sulfate and the solvent was evaporated. The crude product was purified by column chromatography on silica gel with hexane: ethyl acetate (150:1 v/v) as an eluent to afford 3 (1.89 g, 50%). 1H NMR (400 MHz, CDCl3), δ 7.86 (t, 4H), 7.80 (d, 4H), 7.72 (dd, 6H), 7.50 (d, 2H), 7.39 (td, 4H), 5.45 (td, 4H), 4.92 (dd, 8H), 3.38 (dt, 4H), 2.95–2.77 (m, 8H), 2.20 (d, 4H), 1.80–1.61 (m, 4H), 1.45–1.10 (m, 10H), 0.95 (s, 2H). MS (MALDI-TOF) m/z: M+ calcd: 981.01, found: 980.7. Synthesis of OF-Si. Compound 3 (1.89 g, 1.44 mmol) was dissolved in CHCl3, and then a solution of trimethylamine (Me3N) in ethanol (30 wt%, 5 mL) was added. The reaction mixture was stirred for 48 h at room temperature. The solvent was evaporated and the product was dried under vacuum and denoted as compound 4. The obtained product (0.70 g, 0.75 mmol) was added to anhydrous chloroform (50 mL), followed by addition of HSi(OMe)3 (0.77 mL, 6.05 mmol) and a catalytic 6


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amount of Pt(dvs). The mixture was stirred at 60 °C for 12 h. The solvent was evaporated and the obtained solid was purified by column chromatography on silica gel with hexane/ethyl acetate as an eluent to afford OF-Si (0.90 g, 85%). 1H NMR (400 MHz, CDCl3) δ 7.88 – 7.71 (m, 9H), 7.67 (d, 2H), 7.50 (s, 1H), 7.45 – 7.38 (m,3H), 7.31 (ddd, 5H), 3.42 – 2.71 (m, 36H), 2.65 – 2.40 (m, 22H), 2.25 (ddd, 16H), 1.14 (s, 20H), 0.63 (s, 8H). MS (MALDI-TOF) m/z: M+ calcd: 1428.21, found: 1413.2. Preparation of SiO2 NPs. Pure SiO2 NPs were synthesized by a reverse micelle method as reported in the literature.53 Briefly, cyclohexane (7.6 mL), 1-hexanol (1.8 mL) and Triton X-100 (1.8 mL) were mixed rigorously in a clean 50 mL round-bottom glass flask, to which water (0.48 mL) was added to form stable reverse micelles. After 10 min, TEOS (0.1 mL) was added to the emulsion, followed by ammonia solution (28–30%, 0.1 mL) as a catalyst. The mixture was stirred at room temperature for 24 h to ensure complete hydrolysis and condensation of TEOS. The micelles were disrupted with acetone, and the NPs were collected by centrifugation at 5000 rpm for 5 min. A redispersion-centrifugation cycle was repeated five times with washing by ethanol to remove the additive, and yield the SiO2 NPs dispersed in water. Preparation of OF/SiO2 NPs. The fabrication of OF/SiO2 NPs was based on the procedure of SiO2 NPs with the addition of OF-Si. Typically, cyclohexane (7.6 mL), 1-hexanol (1.8 mL) and Triton X-100 (1.8 mL) were mixed rigorously, followed by addition of water (0.48 mL) to form stable reverse micelles. After 10 min, a mixture of TEOS (100 µL) with OF-Si solution in chloroform (3.5 mM, 640 µL/256 µL/128 7



µL/64 µL/42 µL/32 µL/26 µL), and ammonia solution (28–30%, 100 µL) were added. The mixture was stirred at room temperature for 24 h. Acetone was added to the mixture, and the NPs were collected by centrifugation at 5000 rpm for 5 min and washed with ethanol at least five times. The OF/SiO2 NPs was finally dispersed in water. According to the initial molar ratio of OF-Si to TEOS, the obtained OF/SiO2 NPs were denoted as OF/SiO2 0.5%, OF/SiO2 0.2%, OF/SiO2 0.1%, OF/SiO2 0.05%, OF/SiO2 0.033%, OF/SiO2 0.025%, OF/SiO2 0.02%. The OF/SiO2 0.05% NPs were used as a representative sample throughout this work owing to its high quantum yield, except for the Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA). Preparation of PFO NPs. PFO NPs were prepared by a reprecipitation method.54 Briefly, the polymer PFO was dissolved in THF in the dark at a concentration of 2 mg/mL. The polymer solution was filtered through 0.22 µm polytetrafluoroethene (PTFE) filters. Then, a 250 µL portion of the polymer solution was rapidly injected into 5 mL of H2O and subjected to ultrasonication for 5 min. Collection and Development of Latent Fingerprints (LFPs). The OF/SiO2 NPs dry powders were collected by a freeze-dry technique. Four kinds of substrates were selected for deposition of the fingerprints, namely, stainless steel, plastic (polycarbonate, PC), yellow tape for packaging, and glass (cover glass). No special treatments were performed on these substrates. All the fingerprints samples on different substrates were prepared based on the 8


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same procedure, and the fingerprints were collected from the same 26-year-old female donor. Before deposition of the fingerprints, the donor thoroughly washed her hands in water, and rubbed her fingers over oily parts of her face and then gently stamped her fingertips on the substrate. To develop the LFPs, small amounts of OF/SiO2 NPs powders were carefully added onto the imprinted substrates and excess of powder was removed under a gentle air flow for 30 s. The bright field optical pictures were recorded under room light using a Sony α-6000 digital camera, while the fluorescent photographs were captured under a hand-held UV lamp with 365 nm excitation. Characterization. 1H NMR spectra were recorded on a 400 MHz AC Bruker spectrometer. Molecular weight was measured on a Bruker Daltonics BIFLEX III MALDI-TOF analyzer in matrix-assisted laser desorption ionization time-of-flight (MALDI) mode, or Bruker Daltonics APEX П FT-ICR analyzer in ESI mode. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VERTEX 70v spectrometer. Thermogravimetric analysis (TGA) was measured with a SDT-Q600 thermogravimetric analyzer. The size distributions and zeta potentials of NPs dispersions were determined by dynamic light scattering (DLS) with a Malvern Zeta-sizer Nano ZS90 at 25 °C. The size distribution was reported as the average diameter. The size and morphology of the NPs was measured with a field emission scanning electron microscope (SEM, ZEISS SUPRA55) and a transmission electron microscope (TEM, Hitachi H-7650). The SEM samples were prepared by dropping a portion of the NPs suspension onto clean silicon wafers. The TEM samples were prepared by drop casting a nanoparticle suspension onto a carbon-coated copper grid. 9



Ultraviolet-visible (UV-vis) absorption and fluorescence emission spectra were measured on a Hitachi U-3900H spectrophotometer and a Hitachi F-7000 fluorescence spectrophotometer, respectively. Time-domain lifetime measurements were performed with an ultrafast lifetime spectrofluorometer (Delta flex) based on time-correlated single-photon counting. The absolute fluorescence quantum yields were determined with a spectrofluorometer (NanologR FluoroLog-3-2-iHR320, Horiba Jobin Yvon) equipped with an integrating sphere. For samples of the OF-Si solution in CHCl3 and OF/SiO2 NPs, the excitation wavelength was set to be 350 nm, with a scattering spectral range at 345–355 nm and an emission spectral range of 365– 680 nm. For the samples of PFO solution in CHCl3 and PFO NPs, the excitation wavelength was set to be 380 nm, with a scattering spectral range of 375–385 nm and emission spectral range of 390–680 nm. Results and Discussions Design Concept. Scheme 1 illustrates the design concept of this work, including preparation of the conjugated oligomer doped silica NPs and procedures for latent fingerprint development. Here, we selected an oligofluorene (OF) with three repeat units as the fluorophore. This simple structure has outstanding emission properties and is widely used as a blue fluorescent material.9,55 To ensure binding of the OF with the silica matrix, silane groups were added to the side chains of two fluorene units. Quaternary ammoniums groups were introduced into the side chain of the remaining fluorene units to increase the hydrophilicity, resulting in our modified oligomer, OF-Si. The dye-doped silica NPs were synthesized by a previously reported reverse 10


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microemulsion method.53 First, cyclohexane, 1-hexanol, water, and the surfactant Triton X-100 were thoroughly mixed to produce a water-in-oil reverse micelle suspension. Then, a tetraethoxysilane (TEOS) solution containing different amount of OF-Si was added as the silica precursor and ammonia was used as a catalyst for hydrolysis and condensation of the silica precursor. The OF-Si was successfully doped into silica NPs by co-condensation between the silane side chains and TEOS. To optimize the content of OF-Si, the initial molar ratio of OF-Si to TEOS was varied from 0.02% to 0.5%. The obtained OF/SiO2 0.05% NPs were used as a representative sample for the following characterization unless otherwise stated. The obtained hybrid fluorescent silica NPs showed a number of remarkable features. Fluorescent NPs with conjugated oligomer content as low as 0.025–0.05 mol% showed photoluminescence quantum yields as high as 97%, indicating that ACQ effects were limited in the particles. Second, the preparation of the OF/SiO2 NPs via reverse micelle method was low in cost, based on inexpensive materials, which may allow the hybrid NPs to be synthesized on a large-scale. Third, the OF/SiO2 NPs showed good biocompatibility, owing to the low toxicity of the silica matrix and conjugated oligomer. Finally, the OF/SiO2 NPs could be used directly for latent fingerprints detection without any further modification or assistance of additional surfactant. As the control example, we firstly tried to prepare OF-Si NPs by reprecipitation method but failed, because of its easy hydrolysis property in aqueous solution. On the other hand, as a representative polyfluorene, poly(9,9-di-n-octylfluorenyl-2,7-diyl) 11



(PFO) is widely used as blue-emissive materials in various fields including biological sensing or imaging and optoelectronic devices,54,56 and more importantly, it is commercially available. PFO is always suffered from some problems such as aggregation induced quenching, easily variable emission spectrum and photo-induced degradation or reaction under continuous irradiation, which bring problems to practical applications.57-60 Thereby, we chosen PFO and PFO NPs for comparison with our OF/SiO2 NPs to illustrate the improvements on the optical properties. The PFO NPs were prepared by a reprecipitation method without any stabilizer or surfactant.

Scheme 1. Schematic illustration of (a) synthesis OF/SiO2 NPs via reverse micelle method and chemical structures used in the process; (b) procedures for fingerprint development with OF/SiO2 NPs. Synthesis and Characterization of OF-Si and OF/SiO2 NPs. OF-Si was prepared in good yields, following the synthetic route shown in Scheme 2. To incorporate the fluorophore into the silica matrix, four silane groups were added to side chains by allylation followed by hydrosilylation. A Suzuki coupling reaction was used to 12


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synthesize the conjugated oligomer 3, which was then treated with trimethoxysilane to introduce the silane groups and afford OF-Si. The chemical structure of OF-Si was confirmed by 1H NMR, mass spectroscopy and FT-IR spectra (Figure S1-S5). The as-synthesized OF-Si was readily soluble in common organic solvents, such as chloroform, dichloromethane, and DMF, but did not dissolve well in acetone, methanol, ethanol, or isopropanol.

Scheme 2. Synthesis route of OF-Si. The size and morphology of pure SiO2 NPs, OF/SiO2 0.05% NPs and PFO NPs were investigated by transmission electron microscope (TEM) and scanning electron microscope (SEM) observations and dynamic light scattering (DLS) measurements. The pure SiO2 NPs featured a spherical shape with an average diameter of approximately 37 nm, as revealed by the TEM and SEM results (Figure S6). After incorporating the silane-modified conjugated oligomer into the NPs, we observed no differences in the particle shape; however we found a slight increase of the mean particle size, to approximately 45 nm for the OF/SiO2 NPs, as measured from the images in Figure 1a and 1b. The TEM and SEM images showed that both the SiO2 13



and OF/SiO2 NPs were well dispersed and distributed, indicating good control of the particle size and size distribution by the reverse micelle synthesis method. The DLS results revealed an increase of the hydrodynamic radius in water for both the NPs, with the diameter approximately 51 and 66 nm for the SiO2 (Figure S6c) and OF/SiO2 NPs (Figure 1c), respectively. Zeta potential measurements indicated that both NP samples featured a strongly negatively charged surface, with values of -35.9 ± 2.0 mV and -46.1 ± 1.3 mV for the SiO2 and OF/SiO2 NPs, respectively. The negative charge could be attributed to the presence of many hydroxyl groups on the particle surfaces, which also contributed to the colloid stability of the dispersions. The quaternary ammonium salts on the OF-Si structure had little effect on the overall NPs’ surface charge because of the low oligomer content. In the case of the PFO NPs obtained by reprecipitation, the TEM and SEM images (Figure S7) appeared to have a rough spherical shape with a mean size of approximately 90 nm. The size distribution was not as well defined as that of the SiO2 and OF/SiO2 NPs. This effect can be attributed to the formation of organic NPs being sensitive to the preparation conditions used in the reprecipitation process, including the ultrasonication conditions. The DLS results (Figure S7) showed that the PFO NPs had a mean size of approximately 130 nm and Zeta potential measurements indicated a negative surface charge (-36.8 ± 0.6 mV). To verify the incorporation of the conjugated oligomer OF into the silica matrix, we characterized the material with FT-IR spectroscopy and TGA. Considering the detection limits of these two methods, we selected OF/SiO2 0.5% NPs as the test sample owing to its high loading of OF. The results of FT-IR analysis of pure SiO2 14


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NPs, OF-Si, and OF/SiO2 0.5% NPs are shown in Figure 1d. For pure the SiO2 NPs, wide and intense bands at 1101 cm−1 and 3426 cm−1 were found, assigned to an asymmetric vibration of Si–O–Si and a stretching vibration of O–H, respectively. A characteristic peak was found at 2924 cm−1 for the OF-Si sample attributed to stretching vibrations of C–H on the aromatic backbone. In the case of OF/SiO2 0.5% NPs, peaks from both SiO2 and OF-Si were observed, indicating successful incorporation of the OF. TGA curves, shown in Figure 1e, indicated an additional 4.2% weight loss for OF/SiO2 0.5% NPs compared with that of the SiO2 NPs. This weight difference could be attributed to the thermal degradation of the organic conjugated oligomer. These results suggest that the OF was embedded into the silica matrix.

Figure 1. (a) TEM images, (b) SEM images, (c) size distribution and zeta potential (ξ) of OF/SiO2 0.05% NPs. (d) FT-IR spectra of pure SiO2 NPs (black curve), OF-Si (pink curve), and OF/SiO2 0.5% NPs (blue curve). (e) TGA results of pure SiO2 NPs (black curve) and OF/SiO2 0.5% NPs (blue curve). Photophysical Properties of the As-Prepared OF/SiO2 NPs. The photophysical 15



properties of the precursors, OF-Si and PFO, were characterized in chloroform solutions, whereas the OF/SiO2 NPs and PFO NPs were examined in their aqueous dispersions. Figure 2 shows normalized absorption and emission spectra of the obtained NPs, with corresponding details summarized in Table 1. As expected, the pure SiO2 NPs showed no obvious absorption or emission owing to their optical transparency over this wavelength range. The OF-Si sample in CHCl3 exhibited an absorption maximum at approximately 354 nm, corresponding to the π→π* transition of the fluorene chromophore.55 Excitation at 350 nm, gave rise to a structured emission band from 370 to 500 nm with two emission maxima at 398 and 417 nm, similar to that previously reported for OFs.9,55 For the OF/SiO2 0.05% NPs, we observed similar absorption and emission behaviors. As the OF precursor concentration was varied from 0.5% to 0.02% the maximum absorption and emission position of the OF in the silica matrix remained stable. This result indicates that no notable aggregation of OF molecules occurred and that OF was homogenously dispersed in the silica matrix. The control sample, based on PFO polymer NPs, showed somewhat complex behavior. Figure 2c shows the PFO polymer in CHCl3 solution with an absorption peak at 392 nm, which appeared at longer wavelength compared with that of OF owing to its longer conjugated length. After incorporation into the PFO NPs, the maximum absorption wavelength around 401 nm featured a 9 nm red-shift and showed additional structure owing to the formation of a β-phase conformation.59,60 Upon excitation at 380 nm, the emission spectrum of PFO NPs showed a 20 nm 16


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red-shift compared with that of PFO solution, as shown in Figure 2d. Such red-shifts have been widely reported for conjugated polymer NPs and solid films, and are attributed to chain collapse, which results in red-shifted aggregate species, changes of the energetic ordering, and multiple energy transfer.56,57

Figure 2. Normalized absorption spectra (a) and emission spectra (b) of pure SiO2 NPs in water (black curve), OF-Si in CHCl3 (pink curve), and OF/SiO2 0.05% NPs in water (blue curve). Normalized absorption spectra (c) and emission spectra (d) of PFO solution in CHCl3 (orange curve) PFO NPs in water (green curve). Inset in Figure 2c is the chemical structure of PFO. To determine the emission efficiency of the obtained fluorescent NPs, absolute fluorescence quantum yields were measured with the use of a spectrofluorometer equipped with an integrating sphere, and the results are summarized in Table 1. When OF-Si was dissolved in a good solvent, such as CHCl3, the QY was determined to be 17



71.2%, suggesting the strong fluorescent characteristics of the OF conjugated backbone. The QY result for OF was also comparable to those of previous reports, indicating its complete dissolution.9,55 After incorporation into the silica NPs, OF/SiO2 0.5% NPs and OF/SiO2 0.2% NPs showed decreased QY of 62.4% and 69.9%, respectively. The decrease of these values can be attributed to some slight aggregation or close contact between chromophores in the silica matrix, which induced energy transfer and self-quenching. As the molar ratio of OF was reduced to 0.1%, the QY increased and reached 84.3%. When the molar ratio of OF was maintained in the rage of 0.025%–0.05%, the QYs of the fluorescent NPs were as high as 97%, even much higher than OF-Si in solution (71.2%). This high QY could be attributed to the homogenous dispersion of OF in the silica matrix, which efficiently separated each fluorophore and prevented aggregation induced energy losses and self-quenching. Furthermore, compared with the conjugated oligomer at well-solved state, the rigid silica matrix restricted motion of the molecules, reducing free rotation and vibration of the molecules, which decreased nonradiative decay of the excited state. Accordingly, the enhanced radiation process gave rise to higher quantum yields than that in solution. If the amount of OF in the hybrid NPs was further decreased to 0.02%, the QY decreased to 65.7% possibly owing to greater error in the measurement at lower light intensities. Conversely, the PFO NPs showed a QY value of 51.8%, which was lower than that of the free polymer chains in good solvent (63.1%). This reduction in emission efficiency was attributed to aggregation induced depopulation of excitons resulting in 18


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quenching.18,19 The QY of the as-prepared PFO NPs was higher than that reported for PFO NPs (40%),56 owing to residual THF in the mixture. Table 1. Photophysical parameters of the obtained samples including the maximum absorption (λabs) and emission wavelength (λemi), absolute fluorescence quantum yield (ΦF), average fluorescence lifetime (τ). λabs (nm) OF-Si 354 OF/SiO2 0.5% NPs 354 OF/SiO2 0.2% NPs 354 OF/SiO2 0.1% NPs 354 OF/SiO2 0.05% NPs 354 OF/SiO2 0.03% NPs 354 OF/SiO2 0.025% NPs 354 OF/SiO2 0.02% NPs 354 PFO 392 PFO NPs 401

λemi (nm) ΦF (%) τ (ns) solvent 398, 417 71.2 0.64 CHCl3 400, 418 62.4 H2 O 400, 418 69.9 0.95 H2 O 400, 418 84.3 H2 O 400, 418 97.3 0.95 H2 O 400, 418 93.2 H2 O 400, 418 97.7 H2 O 400, 418 65.7 0.92 H2 O 418 63.1 0.52 CHCl3 438 51.8 0.36 H2 O

To further understand the change in emission efficiency, the fluorescence lifetimes of the samples were examined by time-correlated single-photon counting. Fluorescence decay curves are shown in Figure 3. The calculated average lifetimes are listed in Table 1, and the detailed information are displayed in Table S1. The average lifetime of OF-Si in CHCl3 was 0.64 ns, which was consistent with results for similar OF compounds.55 Figure 3a shows that the emission at 418 nm of the OF in the NPs decayed slower than that of free OF in solvent; the lifetime was prolonged from 0.64 ns to 0.95 ns. However, the emission of the PFO NPs showed faster decay than that in solvent, with a shorter lifetime of 0.36 ns, which was close to the value of 0.27 ns previously reported for PFO NPs.56

19



Figure 3. (a) Decay curves of 418 nm emission wavelength of OF-Si in CHCl3 (pink dots) and OF/SiO2 in H2O (blue dots). (b) Decay curves of 418 nm emission wavelength of PFO in CHCl3 (orange dots), and 438 nm emission of PFO NPs in H2O (green dots). It has been reported that fluorene-based compounds and polymers with alkyl substituents at the bridged C-9 position easily degrade under photoirradiation.58 To test the photostability of our materials, the OF/SiO2 and PFO NP dispersions were exposed to continuous excitation from a xenon lamp at 350 nm and 390 nm, respectively, for 5400 s (Figure 4a). The photoluminescence intensity of the PFO NPs rapidly decreased by approximately 25% in the first 500 s then decreased slowly over the remaining time to give a final 36% reduction. We observed a slight recovery in the intensity during the period of 2500–4000 s. This fluctuation was likely caused by a photoinduced reaction of the PFO chains upon exposure to light irradiation.58 However, in the case of the OF/SiO2 NPs, the fluorescence intensity maintained its initial level and only a slight 4% reduction was observed over the tested time range. Thus, we conclude that the conjugated oligomers encapsulated in the silica matrix were not easily photobleached. This result can be reasonably explained by the 20


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restricted movement of the organic molecules embedded in the rigid silica matrix and protection of the fluorophores from external perturbations, such as photo-generated radicals or reactive oxygen species. Fluorescence emission is sensitive to the surrounding environment, including factors such as solvent polarity, temperature and pH. Thereby, we determined the emission intensity of the obtained NPs over a broad range of pH and temperatures, and in different solvents. Figure 4b shows that both the OF/SiO2 NPs and PFO NPs showed good emission stability in the pH range 2–11, with only slight variations in the fluorescence intensity ratio. Figure 4c shows the behavior of the intensity ratio of the OF/SiO2 NPs and PFO NPs as a function of temperature from 5 to 60 °C. I0 was the intensity of the sample at 5 °C. As the temperature was increased, the intensity ratio of the PFO NPs decreased resulting in a 40% reduction at 60 °C. Conversely, the intensity ratio of the OF/SiO2 NPs remained stable with almost no detectable fluorescence loss, indicating that the emission properties were not strongly influenced by temperature. Silica NPs are tolerant to various solvents and the synthesized OF/SiO2 NPs could be dispersed in many common solvents. Hence, we examined the effects of solvent on the emission of OF/SiO2 NPs, as shown in Figure 4d. The PFO NPs could not be examined in different solvents, because the presence of organic solvent induced swelling or dissociation of the NPs. Figure 4d shows the photo- and colloidal- stability of the OF/SiO2 NPs in common solvents, except in hexane. The reduction in fluorescence intensity in OF/SiO2 NPs was attributed to the low polarity of hexane, which lowered the dispersion of OF/SiO2 NPs in it and induced formation 21



of large aggregates and precipitated finally. On the basis of these results, we concluded that the OF/SiO2 NPs had excellent optical stability and were unaffected by photobleaching, solution pH, temperature, or solvent environment.

Figure 4. Photostability of OF/SiO2 0.05% NPs and PFO NPs as a function of (a) light exposure time (0–5400 s), (b) pH values (pH 2-11), and (c) temperature (5– 60 °C). (d) Photostability of OF/SiO2 0.05% NPs in various common solvents. I0 was the intensity of the sample at 0 s, or at pH = 7, or at 5 °C, or in H2O, respectively, for each measurement. Development of Latent Fingerprints with OF/SiO2 NPs. We applied the high brightness and stability of the OF/SiO2 NPs to latent fingerprint detection. An aqueous dispersion of the OF/SiO2 NPs was freeze-dried to obtain powders, which could be used as a fluorescent dusting material. After drying out from the solution, the absorption spectrum OF/SiO2 NPs showed a 11 nm red-shift, accompanied with 22


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appearance of a weak broad absorption band around 490 nm, indicating some slight aggregation between adjacent OF molecules (Figure S8). On the other hand, the detailed structure of its fluorescence spectrum disappeared, leaving a main emission peak around 412 nm. The absolute quantum yield of the OF/SiO2 powders was determined to be 62.2%, which is high enough for practical application. For application, one superior advantage of OF/SiO2 NPs over PFO NPs is their good colloidal stability and facial manipulation. OF/SiO2 NPs can be easily collected by centrifugation or lyophilization, and re-dispersed in various solvent. By contrast, the PFO NPs cannot be centrifugated and re-dispersed due to the poor colloidal stability of organic nanoparticles, and its concentration was lower than 0.1 mg/mL, which means it is hard to accumulate the required amount for optical properties measurement and latent fingerprints development. Thus, for the development of latent fingerprints, only OF/SiO2 NPs was used and evaluated. For the proof-of-concept application in latent fingerprints detection, we selected four types of substrates commonly handled in daily life, namely, stainless steel, glass, plastic (polycarbonate, PC), and yellow tape. The fingerprints from a 26-year-old female donor were deposited on each substrate. Fingers generally are a coated with lipid from sebaceous.50 Fingerprint residue is composed of various compounds, such as nonpolar lipids, salts, amino acids, and proteins. In this study, the collected fingerprints were reasonably supposed to be lipid fingerprints, considering the development procedure (described in experimental section).

Bright field optical images of these fingerprints were taken by digital camera under 23



room light and displayed in Figure 5a. It was hard to get a clear vision of a complete fingerprint pattern using naked eyes, indicating their latent properties on provided substrates. Upon excitation by a hand-held UV lamp with 365 nm excitation, no detectable fluorescence was found and autofluorescence background was proved to be absent in this case. After applying the OF/SiO2 NPs powder to the substrate, excess powder was blow off under an air flow for 30 s. Bright field optical images of these developed fingerprints were captured under room light. Figure 5b shows that the fingerprints patterns on smooth substrate, including stainless steel, glass, plastic PC, and yellow tape, were quite clear and displayed distinct ridge lines. These results reveal the efficient adsorption of OF/SiO2 NPs in the hydrophobic regions of fingerprints. To explore the factor which promoted the adsorption of these nanoparticles, the pure SiO2 NPs were synthesized through a classical Stöber method and used to develop the fingerprints (Figure S9), which also had a comparative ability of turning latent fingerprints to visible ones (Figure S10). Figure S11 shows that the SiO2 NPs prepared by reverse micelles method had an enhanced adsorption efficiency onto the fingerprint patterns. It has been reported that the mechanical adhesion of powder to the moisture and oily components of the fingerprint ridges is mainly guided by the pressure deficit mechanism, and thus small particles adhere to the ridges more efficiently than larger ones.61 The mean sizes of SiO2 NPs (Stöber method), SiO2 NPs (reverse micelles method), and OF/SiO2 NPs were 123 nm, 51 nm, and 66 nm, respectively. Thus, SiO2 NPs (reverse micelles method) and OF/SiO2 NPs displayed enhanced adsorption efficiency onto the fingerprint patterns compared to SiO2 NPs 24


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(Stöber method), due to their smaller sizes. Based on these observation, we can conclude the affinity of OF/SiO2 NPs towards fingerprints mainly roots in the intrinsic feature of SiO2 NPs and its small size, which preferentially adhered to the ridges, leaving the space between ridges non-fluorescent. Upon excitation by 365 nm UV lamp, all the images of the latent fingerprints on different substrates developed by OF/SiO2 NPs exhibited clear fluorescent fingerprint patterns. The fluorescent images of the fingerprints maintained sufficiently high-resolution and contrast between the ridges and spaces. Thus, we confirmed good imaging quality on each kind of substrate with the OF/SiO2 NPs.

Figure 5. Bright field optical images under room light and fluorescent images under 25



UV light (365 nm) of latent fingerprints (a) before and (b) after incorporation of OF/SiO2 0.05% NPs on different substrates including stainless steel, glass, plastic (polycarbonate, PC), and yellow tape. Fluorescent fingerprint patterns of other fingers, including the thumb, index finger, middle finger, and ring finger were also well resolved with the use of the OF/SiO2 NPs (Figure 6a). On the basis of the high resolution of the fingerprint images acquired, digital magnification of the fingerprints was performed to extract greater detail and personal features. Figure 6b shows a representative fluorescent fingerprint image developed on yellow tape together with four expanded images. The expanded images allowed small features, including terminations, islands, bifurcation, and cores, to be clearly identified. In addition, the fluorescence intensity of these developed fingerprints declined slightly after long-term storage over 5 months (Figure S12).

Figure 6. (a) Fluorescence images of latent fingerprints from different fingers (thumb, index finger, middle finger, and ring finger) developed with OF/SiO2 0.05% NPs on 26


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plastic (PC). (b) Fluorescence images of latent fingerprints developed from OF/SiO2 0.05% NPs on yellow tape, and magnified regional images with specific features including a termination 1, an island 2, a bifurcation 3, and a core 4. Our results show that the latent fingerprint patterns developed from the OF/SiO2 NPs allowed clear imaging of fingerprints on various types of substrate. The well-resolved features will be useful for discerning personal identifying features. Although a quantity of emissive materials based on SiO2 NPs has been reported as fluorescent probes for efficient latent fingerprints detection,62-65 OF/SiO2 NPs still show some advantages. Firstly, the high emission efficiency and photostability against various environmental variation ensure them promising material in practical application. Secondly, the fluorophores were attached to the silica matrix through chemically stable covalent bonds, avoiding some possible leak and loss of these fluorophores during the storage or usage process. Furthermore, silica NPs and conjugated oligomers are regarded as nontoxic and biocompatible materials. Hence, we believe that our conjugated oligomer doped fluorescent silica NPs, OF/SiO2 NPs, will be safe and effective in practice use for latent fingerprint detection. Conclusions In this study, we report the incorporation of a conjugated oligomer into silica NPs to obtain highly efficient and stable fluorescent nanomaterials. The fluorescence quantum yields of the obtained hybrid NPs were as high as 97% when a conjugated oligomer precursor was added at a molar content of 0.025%–0.05%. These fluorescent NPs also showed good photostability under UV light irradiation, and excellent 27



resistance to a wide range of pH, temperatures, and different solvents. The enhanced emission efficiency and photostability could be attributed to the fixation of the conjugated oligomer fluorophore in the rigid silica matrix, which reduced rotation and vibration of the molecules and provided protection from external perturbations. The obtained OF/SiO2 NPs were found to be efficient agents for optical/fluorescent detection of latent fingerprints.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications websites at DOI: Additional characterization data of synthesized compounds (NMR and FT-IR spectra), characterization data of SiO2 and PFO nanoparticles (TEM, SEM, and DLS results), fingerprints images developed by pure SiO2 nanoparticles, evolution of fluorescent images of developed fingerprints with storage time, and detailed analysis of fluorescence decay experiments (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L.L.), [email protected] (Q.C.) Author Contributions

†S.Z. and R.L. contributed equally to this study. Notes The authors declare no competing financial interest. 28


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Acknowledgments This work was supported by the National Natural Science Foundation of China (51503015, 51373022), the State Key Laboratory for Advanced Metals and Materials (2016Z-08), and the Fundamental Research Funds for the Central Universities (FRF-BR-16-021A).

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