Polyethylenimine-Derived Fluorescent Nonconjugated Polymer Dots

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Polyethyleneimine-Derived Fluorescent Non-Conjugated Polymer Dots with Reversible Dual-Signal pH-Response and Logic Gates Operation Shi Gang Liu, Ting Liu, Na Li, Shuo Geng, Jinglei Lei, Nian Bing Li, and Hong Qun Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12695 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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The Journal of Physical Chemistry

Polyethyleneimine-Derived Fluorescent Non-Conjugated Polymer Dots with Reversible Dual-Signal pH-Response and Logic Gates Operation Shi Gang Liu,† Ting Liu,† Na Li,† Shuo Geng,† Jing Lei Lei,‡ Nian Bing Li,*, † and Hong Qun Luo *,† †

Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry

of Education), School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P.R. China ‡

School of Chemistry and Chemical Engineering, Chongqing University, Chongqing

400044, P.R. China

*

Corresponding Authors

*

Nian Bing Li

*

2, Tiansheng Road, BeiBei District, Chongqing, 400715, China. Tel: +86 23

68253237; fax: +86 23 68253237; E-mail address: [email protected] *

Hong Qun Luo

*

2, Tiansheng Road, BeiBei District, Chongqing, 400715, China. Tel: +86 23

68253237; fax: +86 23 68253237; E-mail address: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT A kind of non-conjugated polymer dots possessing autofluorescence was synthesized

by

hyperbranched

polyethyleneimine

and

salicylaldehyde

via

environmentally friendly imine crosslinking and self-assembly in the aqueous phase. Both fluorescence and ultraviolet-visible (UV-vis) absorption of the polymer dots showed a sensitive and reversible response to pH. The mechanism of pH-dependent optical properties was investigated. The existence of two proton-responsive functional groups (phenolic hydroxyl groups and amine groups) on the polymer dots surface is crucial. The pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV-vis absorption is induced by ionization of phenolic hydroxyl groups. Based on the pH-mediated optical properties, colorimetric and fluorescent dual-signal pH sensor and multiple logic gates were developed. The fluorescent non-conjugated polymer dots with easy preparation and sensitive response to pH are very promising for applications in biological, pharmaceutical, and material science fields.

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INTRODUCTION Over the past decades, a variety of new fluorescent materials, such as semiconductor quantum dots (QDs),1 aggregation-induced emission (AIE) dyes,2 gold or silver nanoclusters (NCs),3,4 grapheme quantum dots (GQDs),5,6 and carbon dots (CDs),7-9 have been created for application in many areas, for example, bioimaging, fluorescent sensors, solar cell, and light-emitting diodes (LEDs). Among these fluorescent materials, conjugated polymer dots (CPDs) have attracted a great deal of interest because they are endowed with some intrinsic advantages such as high brightness, good biocompatibility, and excellent photophysical properties.10 Up to now, numerous conjugated polymer dots have been developed for sensors, fluorescence imaging, gene and drug delivery, and so on.11-13 The technical routes for the preparation of conjugated polymer dots mainly include mini-emulsion method, reprecipitation, and microfluidic technology, which are generally based on self-assembly of a fluorescent conjugated polymer.10,14,15 Even though great achievements have been obtained in the preparation of conjugated polymer dots, some disadvantages still exist. For example, the preparation is usually operated in organic solvents and most conjugated polymers need sophisticated multistep synthetic pathways, which are unpleasant and do not embrace the principles of green chemistry. On the one hand, thus the development of conjugated polymer dots with easy and eco-friendly preparation is still in pursuit. On the other hand, fortunately, in very recent years a few kinds of non-conjugated polymer dots (NCPDs) with strong fluorescence emission have been discovered.16-19 Non-conjugated polymer dots are

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always constructed from non-conjugated polymers which are not similar to conjugated polymers possessing continuous conjugated main chain or π-aromatic backbones and delocalized electronic structure. Usually, their preparations are facile and environmentally friendly because of the use of water-soluble precursors. Meanwhile, these non-conjugated polymer dots have ample surface functional groups and favorable optical properties. Thus they are very competitive as a novel class of fluorescent materials. However, due to the lack of typical fluorescent units, the mechanism of their intrinsic fluorescence is not well understood and it is still a very open debate among researchers. The research of fluorescent non-conjugated polymer dots is in its infant stage. The synthesis of new non-conjugated polymer dots and studies of their fundamental properties are very significant. Herein, we report an environmentally benign method to synthesize a kind of fluorescent non-conjugated polymer dots. The polymer dots were constructed by hyperbranched polyethyleneimine (PEI) and salicylaldehyde via Schiff base reaction and self-assembly (Scheme 1A). The preparation of the PEI-salicylaldehyde polymer dots (S-PEI PDs) is operated in water at room temperature (25 oC), which is very facile in comparison with most reported methods for fluorescent polymer dots. The fluorescence origin was discussed and also we found that fluorescence and UV-vis absorption of the S-PEI PDs showed sensitive and reversible response to pH (Scheme 1B). The mechanism of the pH-responsive optical properties was investigated in detail. Two proton-responsive functional groups (phenolic hydroxyl groups and amine groups) on the polymer dots surface are responsible for the dual-signal pH response.

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The pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV-vis absorption is induced by ionization of phenolic hydroxyl groups. Wide pH-responsive linear ranges have been obtained with both colorimetric and fluorescent method. As a result, the S-PEI PDs can be used as a versatile nanoprobe for pH sensing in aqueous media. Besides, multiple logic gates, including a dual-output INHIBIT (INH) logic gate, were constructed based on the pH-mediated dual optical signals. In recent years great efforts have been made to replicate various Boolean logic gates on the basis of a diversity of materials (for example, organic molecules, semiconductor quantum dots, gold nanoparticles, sol-gel, polymeric materials, and DNAzymes) employing a single signal such as UV-vis absorption, fluorescence, electrochemical signal, or morphology, as output.20,22 However, using one material with dual or multiple signals as output to mimic logic gates operation is very challenging and has seldom been reported.

Scheme 1. Schematic illustration of the synthesis of S-PEI PDs (A) and the mechanism of pH-responsive UV-vis absorption and fluorescence (B). 5



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EXPERIMENTAL SECTION Materials. Salicylaldehyde and benzaldehyde were supplied by Chengdu Kekong

Chemical

Reagent

Co.,

Ltd.

(Sichuan,

China).

Hyperbranched

polyethyleneimines (PEI) with different molecular weights (Mw = 1800 and 10 000) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Other reagents not mentioned here were of analytical reagent grade, and all the chemicals were used as received. Ultrapure water with a resistivity of 18.2 MΩ cm was used for the experiments. As a wide buffer system, Britton-Robinson (BR) buffer solutions (0.04 M) with various pH values were prepared in accordance with standard protocols and used. Instruments. The fluorescence spectra were collected with an F-2700 spectrofluorometer (Hitachi, Japan) equipped with a 150 W xenon lamp. The voltage of photomultiplier tube (PMT) was set at 400 V, and the slit width was 10 nm for both excitation and emission. A UV-2450 spectrophotometer (Shimadzu, Japan) was used for recording UV-vis absorption spectra. The Fourier transform infrared (FT-IR) spectra were tested by the use of a Bruker IFS 113v spectrometer (Bruker, Germany) after pelleting S-PEI PDs fine powder with KBr. Nuclear magnetic resonance (NMR) spectra were obtained on a Bruker AVANCE III 600 (600 MHz) (Bruker, Germany). The micro-morphology of S-PEI PDs was investigated using a JEM 1200EX transmission electron microscope (TEM) (JEOL, Japan) operating at 200 kV. Thermogravimetric analysis (TGA) was performed using an SDT-Q600 simultaneous thermal analyzer (TA Instruments, USA) with a heating rate of 20 °C/min in N2 6


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atmosphere from 0 to 800 °C. Zeta (ζ) potential measurements were performed on a Zetasizer Nano-ZS90 (Malvern Instruments Ltd., U.K.). Fluorescence lifetime decays were taken on an Edinburgh FL 920 fluorescence spectrometer (Edinburgh, U.K.) with a Xe lamp as the excited light source. A PHS-3C pH meter supplied by Shanghai Leici Instrument Co., Ltd. (Shanghai, China) was used to detect pH values of solutions. Preparation of S-PEI Polymer Dots. The S-PEI PDs were created by PEI and salicylaldehyde via environmentally friendly imine crosslinking and self-assembly (Scheme 1A). Firstly, 100 µL of 0.1 g mL-1 PEI (Mw = 10 000) was dissolved in 4.87 mL of water by stirring for about 1 min, and then 30 µL of salicylaldehyde (99 wt %) was added. Then, the mixture was stirred at 25 oC for 4 h. Subsequently, the as-prepared yellow S-PEI PDs solution was dialyzed against ultrapure water through a dialysis bag (molecular weight cutoff of 1000 Da) for 48 h. The product inside the dialysis bag was collected and subjected a vacuum-rotary evaporation procedure to obtain a viscous material which was further dried under vacuum at 55 ºC for 6 h. Then a 1.6 mg mL-1 S-PEI PDs solution was prepared as the stock solution by redissolving the S-PEI PDs with ultrapure water via ultrasonic treatment, and it was stored at 4 oC for further study. The preparation of PEI-benzaldehyde polymer dots is described simply as follows: 100 µL of 0.1 g mL-1 PEI (Mw = 10 000) was dissolved in 4.85 mL of ethanol, and then 50 µL of benzaldehyde (99 wt %) was added. Subsequently, the mixture was stirred at 90 oC for 4 h. Then, the resulting product was dialyzed against ethanol 7



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through a dialysis bag for 48 h. Other procedures were the same as the preparation of S-PEI PDs. Quantum Yield Measurement. The quantum yield of the S-PEI PDs in water was determined by a slope method.23 Quinine sulfate in 0.1 M H2SO4 (φ = 54%) was chosen as a standard to calculate the quantum yield in this work. The following equation (1) was used to calculate the quantum yield: φx = φst (Kx /Kst )(ηx /ηst )2

(1)

where φ is the quantum yield, K is the slope of integrated fluorescence intensity against absorbance, and η is the refractive index. The subscript “st” refers to the standard, and “x” refers to the samples. pH Response Experiments. Briefly, 950 µL of BR buffers with various pH values, and 50 µL of 1.6 mg mL-1 S-PEI PDs were mixed together, and then the fluorescence and UV-vis absorption spectra of the mixtures were recorded on a spectrofluorometer and UV-vis spectrophotometer. The excitation wavelength was 370 nm in all of the experiments except specially pointed out. For the pH reversible measurements, 1 mL of 0.08 mg mL-1 S-PEI PDs with pH 10.0 (adjusted by 0.01 M NaOH) was used as an initial solution, and then 1 µL of 1 M HCl and 1 µL of 1 M NaOH were alternately added to the mixture before every fluorescence and UV-vis spectrum measurement.

RESULTS AND DISCUTION

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Preparation and Characterization of S-PEI Polymer Dots. Commercial hyperbranched PEI, which contains abundant amino groups, was modified by salicylaldehyde via Schiff base reaction under ambient conditions (25 oC) in aqueous solution. The covalent conjugation of salicylaldehyde onto PEI was confirmed by Fourier transform infrared (FT-IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. As shown in Figure S1 in the Supporting Information, curves a and b show the FT-IR spectra of PEI and S-PEI PDs, respectively. Several feature absorption bands of PEI at the region from 1283 to 1593 cm−1 in PEI associated with the stretching vibration of N-H bond, and the absorption peaks at 2940 and 2810 cm−1 corresponding to the stretching vibration of CH2 bonds are found. Compared to the FT-IR spectrum of PEI, another obvious new peak at 1632 cm−1 is found in the S-PEI PDs spectrum, which is assigned to the C=N bond.24,25 The S-PEI PDs have wide absorption centred at 3410 cm−1 which is an overlap of absorptions of O-H bonds and N-H bonds, and the absorption bands within 900-600 cm−1 are caused by the bending vibration of aromatic rings. Figure S2A and S2B display 1H NMR spectra of PEI and S-PEI PDs, respectively. A new peak at 8.00 ppm belonging to N=CH protons24 and two peaks centred at 6.75 and 7.30 ppm corresponding to protons of aromatic rings on the 1H NMR spectrum of S-PEI PDs are observed. In addition, the thermogravimetric analysis reveals that the dramatic decomposition of the S-PEI PDs occurs at about 210 o

C (Figure S3), which is apparently different from that of PEI (about 340 oC).26 The

results demonstrate the successful conjugation of salicylaldehyde onto PEI. Figure 1A is a transmission electron microscope (TEM) image and reveals that 9



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the S-PEI PDs are monodisperse with near-spherical shape and a mean diameter about 6 nm. Figure S4 is a high resolution TEM image of the polymer dots, which reveals that the polymer dots are amorphous. The size of the S-PEI PDs is admirable, for previous research shows that polymer dots with small size (< 10 nm in diameter) are desirable for certain applications, for example, biological tracing.27 Remarkable Tyndall phenomenon can be observed when the S-PEI PDs solution was radiated by a laser pointer (Figure 1B). The formation of the polymer dots can be attributed to the following factor. After imine crosslinking, hydrophobic aromatic nucleus and C=N bonds were incorporated to the PEI, which made the hyperbranched structure of PEI have to fold and shrink, then self-assembling into uniform polymer dots in water.28 However, the S-PEI PDs show favorable dispersity in aqueous media, which can be ascribed to the contribution of those hydrophilic groups on the surface of the polymer dots, like phenol hydroxyl groups and amine groups. The S-PEI PDs solution can keep stability for more than three months when stored at the condition of 4 oC. It is known that Schiff base reaction can be performed both in water and in organic solutions under mild reaction conditions and also involves only water as a byproduct.29 Both PEI and salicylaldehyde have good water solubility and high reactivity, and thereby, the synthesis for the S-PEI PDs was carried out in an aqueous solution at room temperature (25 oC). Thus, the preparation of fluorescent polymer dots reported here is very facile and environmentally friendly compared with other methods, for example, nano-reprecipitation and emulsion polymerization that are relatively complicated and usually operated in organic solvents.22,23

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Figure 1. TEM image (A) and Tyndall phenomena (B) of S-PEI PDs. Inset of (A) is size distribution graph. Laser pointers of (B): a, green (532 nm); b, red (635 nm). (C) UV-vis absorption spectra of PEI, salicylaldehyde, and S-PEI PDs (pH 8.86). Insets are photographs of S-PEI PDs solution under visible light and UV light (365 nm). (D) Fluorescence emission spectra of S-PEI PDs (80 mg L-1, pH 8.86) under different excitations.

Investigation of Optical Properties. The S-PEI PDs aqueous solution is pale yellow which is different from the colorless solution of sole PEI or salicylaldehyde, and the diluted S-PEI PDs water solution emits blue-green fluorescence radiated with a 365 nm light (inset of Figure 1C). As exhibited in Figure 1C, at above 250 nm of the UV-vis spectra, compared to PEI with insignificant absorbance and salicylaldehyde possessing two absorption peaks located at 254 and 325 nm, the S-PEI PDs water solution has three absorption bands centred at 252, 317, and 398 nm, respectively. Commercial PEI or salicylaldehyde alone has insignificant fluorescence, while the 11



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S-PEI PDs show unique fluorescence properties. With the change of excitation wavelength from 270 to 400 nm, two maximum emission wavelengths were found (Figure 1D), which is different from the fluorescence emission of both excitation-independent organic dyes and excitation-dependent carbon dots.30,31 It can be seen that the S-PEI PDs have a maximum emission at 415 nm with excitation from 270 to 310 nm, whereas the maximum emission converts to 495 nm with the excitation varying from 320 to 400 nm. The maximum excitation wavelengths for emission of 415 and 495 nm are 290 and 370 nm, respectively (Figure S5A and S5B). The fluorescence emission of 415 and 495 nm could be ascribed to the radiative relaxations of the π-π* transitions and the n-π* transitions,32-34 respectively. Figure S6A and S6B show time-resolved fluorescence spectra of the S-PEI PDs at the two emission peaks. The fluorescence lifetime at λex = 290 nm and λem = 415 nm is 4.84 ns, whereas there is a longer fluorescence lifetime (21.36 ns) at λex = 370 nm and λem = 495 nm. The result confirms the intrinsic difference of the two fluorescence emission. As known, the fluorescence of conjugated polymers is derived from large π-conjugated backbones and delocalized electronic structure. Though the S-PEI PDs have not continuous conjugated main chain, the formation of rigid nano-structure may suppress the intramolecular rotations of aromatic nucleus and imine bonds, which makes fluorescence emission possible.35

Generally, imine bonds can be easily reduced by a strong reducing agent such as sodium borohydride (NaBH4) to secondary amines via reaction (2): 36 (2) 12


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However, the aromatic nucleuses are unable to be reduced by NaBH4. To further shed light on the fluorescence origin, we performed an experiment in which the S-PEI PDs were treated by NaBH4. As shown in Figure S7A, the absorption of the S-PEI PDs has remarkable change after reduction by NaBH4, and it can be seen that there is only a single absorption peak at 278 nm above 250 nm. Meanwhile, the fluorescence at 495 nm nearly disappears (Figure S7C), while the fluorescence intensity at 415 nm is increased (Figure S7B). The increase of fluorescence at 415 nm may be caused by the elimination of intramolecular resonance energy transfer. As shown in Figure S8, the absorption spectrum of S-PEI PDs in 350-450 nm is overlapped partly with the fluorescence spectrum near 415 nm. Therefore, an intramolecular resonance energy transfer can occur for the S-PEI PDs when excited by a light of 290 nm. However, after reduction by the NaBH4, the absorption in 350-450 nm disappears, and thus the energy transfer does not take place, accompanied by the fluorescence enhancement. The results indicate that the conjugated structure of aromatic ring and imine bond was broken after the C=N bonds were reduced, accompanied by the change of absorption and fluorescence.

Besides, the fluorescence properties of emission at 495 nm of the S-PEI PDs were further studied because of its relatively lower excited energy and longer emission wavelength. As exhibited in Figure S9A and S9B, the fluorescence intensity shows concentration-dependence and increases in proportion to the S-PEI PDs concentration over the range from 0 to 300 mg L-1. The fluorescence quantum yield of the S-PEI PDs in water was estimated to be 4.6% by the use of quinine sulfate as a 13



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standard reference (Figure S10). Various concentrations of salicylaldehyde were used in the preparation of the polymer dots to investigate its effect on the emission. Figure S11A and S11B show the results, and it can be seen that the maximum emission wavelength does not change obviously although the fluorescence intensity changes.We also explored the effect of molecular weights of PEI on the fluorescence. When PEI with different molecular weights (Mw = 1800 and 10 000) was used, the maximum emission wavelength was constant (Figure S12) and fluorescent intensity increased slightly. Thus we had chosen the PEI with molecular weight of Mw = 10 000 for the preparation of S-PEI PDs. pH-Responsive UV-vis Absorption and Fluorescence. The S-PEI PDs show pH-responsive optical properties. In general, UV-vis absorption spectrum is determined by many elements including molecular structure, solvent environment, and concentration, to name just a few.37-39 In this work, pH has a remarkable effect on the absorption of the S-PEI PDs. Figure 2A displays the variation of UV-vis absorption with different pH values (BR buffer, 0.04 M). On the one hand, the absorption peaks show some shifts in the pH buffers compared with that in pure water. On the other hand, it also can be seen that with the increase of pH, the absorbance at 322 nm (A322) is decreased, while the absorbance at 378 nm (A378) is increased. The change of the absorption even can be recognized by naked eyes because the solution is colorless in acidic solution, but the solution turns to yellow under basic condition. Figure 2B depicts the plots of A322 and A378 as a function of pH values. It can be seen that the response scope of the two absorptions is from pH ~6 to ~11, and the ratio of 14


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A378 and A322 shows a favorable linear relationship (R2 = 0.996) versus pH ranging from pH 7.24 to 9.33. A322 or A378 alone also has a good linear relationship (R2 = 0.992 for A322 and R2 = 0.991 for A378) in the pH range from 7.24 to 10.03 (Figure S13A and S13B). At the same time, the fluorescence is also pH-responsive. The S-PEI PDs solution has higher fluorescence intensity under acidic conditions than that under basic conditions. Figure 2C displays the fluorescence spectra under various pH values. The maximum emission wavelength is constant despite the reduced intensity. Figure 2D shows the relationship of fluorescence intensity with the pH values, and a good linear relationship (R2 = 0.992) versus pH values ranging from pH 5.00 to 8.98 is obtained. Figure 2E and 2F are photographs of the S-PEI PDs in various pH buffers under visible light and a UV light (365 nm), respectively. Additionally, the pH-responsive UV-vis absorption and fluorescence are quite reversible (Figure 3A and 3B) and the response is very fast (within 1 min). The results indicate that the S-PEI PDs can be used as a versatile and sensitive nanosensor with dual-mode optical signal output for pH sensing in aqueous media. The wide fluorescence responsive linear range and ratiometric colorimetric method for pH sensing may make it very competitive and versatile compared to other pH probes with single signal readout.40-42

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Figure 2. UV-vis absorption spectra (A), absorbance at 322 and 378 nm (B), fluorescence emission spectra with 370 nm excitation (C), fluorescence intensities at 495 nm (D), and photographs under visible light (E) and UV light of 365 nm (F) of S-PEI PDs (80 mg L-1) as a function of pH values. Inset of (B) is a linear relationship of A322/A378 versus pH values over the range from pH 7.24 to 9.33, and inset of (D) is a linear relationship of F495 versus pH values over the range from pH 5.00 to 8.98.

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Figure 3. pH-mediated reversibility of UV-vis absorption (A) and fluorescence (B) of S-PEI PDs. pH was tuned alternately between pH 4.50 and pH 10.00.

Next, the mechanism of the pH-mediated optical properties was explored. It is known that PEI, as a hyperbranched cationic polyamine, contains ample primary, secondary, and tertiary amine groups which make it possess a very strong electron-donating ability. Thus, some fluorescent agents that act as electron acceptors can bond to PEI by electrostatic interaction and donor-acceptor interaction. As a result, electron transfer between PEI and the fluorescent agents can take place, accompanied

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by the fluorescence quenching. Based on the principle, PEI has been successfully proposed to the application for fluorescence analysis.43,44 In this work, the pH-dependent fluorescence may be determined by two factors: (1) phenolic hydroxyl groups and (2) amine groups. As is known, phenolic hydroxyl groups would turn to phenoxides in basic solution; and amine groups can transfer electrons to fluorescent units under basic condition, while protonation for most amine groups under acidic condition results in the elimination of electron transfer. To explore the effective factor, we synthesized another polymer dots using PEI and benzaldehyde which lacks phenolic hydroxyl groups. The TEM image and fluorescence excitation and emission spectra are shown in Figure S14A and S14B. Figure S14C and S14D display the absorption spectra and emission spectra of the PEI-benzaldehyde PDs dispersed in different pH buffers (pH 3.14, 7.00, and 9.68). It can be seen that the fluorescence intensity is also reduced along with increasing pH, but the absorption peaks have not remarkable change. The results demonstrate that the reduced fluorescence intensity under basic conditions is because the unprotonated amine groups serve as an electron donor resulting in electron transfer, accompanied by the decrease of fluorescence intensity. To further understand the fluorescence change, we tested zeta potentials of the S-PEI PDs at 25 oC in BR buffers with various pH values (pH 4.57, 7.24, and 10.02). As exhibited in Figure S15, the zeta potential is 15.0 mV in pH 4.57 BR buffer, while it descends to be -5.59 mV when the pH turns to 10.02. The decline of zeta potential with increasing pH validates the different protonation degree of amine groups in various pH buffer solutions and ionization of phenolic hydroxyl groups

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under basic condition. Therefore, it can be concluded that the pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV-vis absorption is induced by ionization of phenolic hydroxyl groups. Thus the developed S-PEI PDs possess two functional motifs that are sensitive to pH (Scheme 1B), and more importantly, the output is independent with different optical signal.

Previous studies show that high ionic strength is of benefit to the protonation of macromolecules containing amines.45-47 Therefore, the S-PEI PDs may be responsive to high ionic strength. As anticipated, high ionic strength has a significant effect on fluorescence intensity. Figure 4A shows the fluorescence enhancement with increasing NaCl concentration in the range of 1 to 200 mM (Figure 4B). But when the concentration of NaCl exceeds 0.2 M, the change of fluorescence intensity is not regular. Interestingly, in the NaCl aqueous solutions, absorbance at 398 nm is increased, and also, the gradual blue-shift of the maximum absorption peak can be observed (Figure 4C). Figure 4D shows the change of absorption peak among 350 to 450 nm and absorbance at the maximum absorption, which is regular with increasing NaCl concentration. It can be noticed that in NaCl aqueous solution both the fluorescence intensity and the absorbance at 398 nm are increased which are different from that in BR buffers in which the absorbance is increased but the fluorescence intensity is decreased with increasing pH values. This is because, in the non-buffer solution, the protonated amine groups facilitate ionization of phenolic hydroxyl groups. Other three salts including KCl, NaNO3, and KNO3 have similar effects on 19



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the optical properties of the S-PEI PDs as shown in Figure S16A-F. These results further confirm that the fluorescence intensity is adjusted by protonation degree of amine groups.

Figure 4. Fluorescence emission spectra with 370 nm excitation (A), fluorescence intensities at 495 nm (B), UV-vis absorption spectra (C), and maximum absorption wavelength and absorbance among 350 to 450 nm (D) of S-PEI PDs (80 mg L-1) versus increasing concentration of NaCl.

Logic Gates Operation. Inspired by the pH-responsive UV-vis absorption and fluorescence, we employed the S-PEI PDs to construct logic systems capable of performing several logic gates. For most reported molecular logic gates, with the drawback of the only single signal as an output, they are just able to perform the single logic operation. In this work, on the basis of pH-mediated multi-signal response, multiple logic systems can be designed by rationally defining logic states. In these 20


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logic operations, the S-PEI PDs aqueous solution served as a gate, while H+ and OHwere used as inputs and optical signals as outputs. The presence and absence of inputs were defined 1 and 0, respectively. The different values of output signals coded with Boolean logic functions as 1 and 0 are illustrated in Figure 5A. At first, we used H+ as input, and the absorbance at 322 nm (A322) and the absorbance at 378 nm (A378) as output, respectively. Simple colorimetric YES and NOT logic gates were fabricated, as exhibited in Figure S17A and S17B. Similarly, using the fluorescence intensity at 495 nm (F495) as output and H+ as input, a fluorescent YES logic gate can be constructed (Figure S17C). Since the optical response to pH is highly reversible, construction of more useful and multiple-configurable logic gates is possible. When H+ and OH- were as chemical inputs, colorimetric INH logic gate (A322 as output) and IMPLICATION (IMP) logic gate (A378 as output) were constructed (Figure 5Ba). Furthermore, a dual-output INH logic gate can be developed, in which A322 and F495 are simultaneously used as output (Figure 5Bb). A logic gate with two output signals may improve its stability greatly, and so far, dual-output logic gates have been barely reported.48 Fluorescent materials, such as semiconductor quantum dots, gold and silver nanoclusters, and carbon dots, are regarded as very promising candidates for the development of logic devices with better applicability for molecular logic compute and biomedical research.49-52 These developed logic systems based on the pH-responsive S-PEI PDs with high sensitivity and fast response may also hold potential applications in the field of material science and biomedicine.

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Figure 5. (A) Definition of logic state for different output signals. (B) Colorimetric INH and IMP logic gates and their truth table (a), and dual-output INH logic gate and its truth table (b).

CONCLUSIONS In conclusion, a new kind of fluorescent non-conjugated polymer dots with good dispersity in water was constructed by hyperbranched PEI and salicylaldehyde via Schiff base reaction and self-assembly. The preparation of the S-PEI PDs is operated in the aqueous phase at room temperature (25 oC) via one-pot reaction, which is very facile and environmentally friendly in comparison with most reported methods for fluorescent polymer dots. The fluorescence and UV-vis absorption spectra of the S-PEI PDs exhibit a sensitive and reversible response to pH. The pH-responsive linear range for the ratiometric colorimetric method is pH 7.24 to 9.33, and that for 22


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fluorescence is pH 5.00 to 8.98, thus the S-PEI PDs can be used as a very competitive and versatile nanoprobe for pH sensing in aqueous media. We also investigated the mechanism of the pH-dependent fluorescence and UV-vis absorption, and the pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV-vis absorption is induced by ionization of phenolic hydroxyl groups. Furthermore, based on the pH-mediated optical properties, several logic gates, such as a dual-output INH logic gate, were developed successfully. The fluorescent non-conjugated polymer dots with ease of preparation and sensitive response to pH may hold great potential for applications in biological, pharmaceutical, and material science fields.

ASSOCIATED CONTENT Supporting Information FT-IR spectra, 1H NMR spectra, thermogravimetric analysis, HRTEM image, dual excitation and emission spectra, time-resolved fluorescence spectra, UV-vis absorption spectra and fluorescence spectra before and after reduction by NaBH4, overlapped spectra, fluorescence properties, linear relationship, TEM image, UV-vis absorption spectra and fluorescence spectra of PEI-benzaldehyde PDs, Zeta potentials, effect of salts on UV-vis absorption spectra and fluorescence spectra, and symbols of YES and NOT logic gates. The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 23



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Corresponding Authors *

E-mail address: [email protected], [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (No. 21675131, 21273174), the Municipal Science Foundation of Chongqing City (No. CSTC-2015jcyjB50001), and the Innovation Foundation of Chongqing City for Postgraduate (CYS16045).

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TOC Graphic

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Polyethylenimine-Derived Fluorescent Nonconjugated Polymer Dots

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