Design of Multiple Logic Gates Based on Chemically Triggered

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Design of Multiple Logic Gates Based on Chemically Triggered Fluorescence Switching of Functionalized Polyethylenimine Yi Pan,† Yupeng Shi,† Zhihua Chen,† Junying Chen,† Mengfei Hou,† Zhanpeng Chen,† Cheuk-Wing Li,‡ and Changqing Yi*,† †

Key Laboratory of Sensing Technology and Biomedical Instruments (Guangdong Province), School of Engineering, Sun Yat-Sen University, Guangzhou, 510275 China ‡ Institute of Chinese Medical Sciences, University of Macau, Macau, China S Supporting Information *

ABSTRACT: In this study, two new functionalized polyethylenimine (PEI), PEIR and PEIQ, have been synthesized by covalently conjugating rhodamine 6G (R6G) or 8-chloroacetyl-aminoquinoline (CAAQ) and have been investigated for their sensing capabilities toward metal ions and anions basing on fluorescence on−off and off−on mechanisms. When triggered by protons, metal ions, or anions, functionalized PEIs can behave as a fluorescence switch, leading to a multiaddressable system. Inspired by these results, functionalized PEI-based logic systems capable of performing elementary logic operations (YES, NOT, NOR, and INHIBIT) and integrative logic operations (OR + INHIBIT) have been constructed by observing the change in the fluorescence with varying the chemical inputs such as protons, metal ions, and anions. Due to its characteristics, such as high sensitivity and fast response, developing functionalized PEI as a new material to perform logic operations may pave a new avenue to construct the next generation of molecular devices with better applicability for biomedical research. KEYWORDS: logic gates, rhodamine 6G, 8-chloroacetyl-aminoquinoline, functionalized polyethylenimine, fluorescence

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such as Cu2+.30−32 However, the nonspecific chelation between amino groups of PEI and metal ions makes the detection selectivity challenged. Fortunately, the abundance of primary amine in PEI structure possesses the ease of chemical modification, and it is thus possible to graft PEI with specific recognition elements, such as biomolecules and organic molecules, to improve the detection selectivity. Through rational design, specific recognition elements conjugated onto PEI can possibly result in the change of fluorescence signals by transferring energy or electrons. Therefore, the development of functionalized PEI-based optical platform for bio- and chemical analysis with higher specificity is feasible. Recently, we have prepared highly sensitive and selective probes for the specific detection and intracellular imaging of Zn2+ by facilely conjugating quinoline derivatives onto the surface of PEI-

INTRODUCTION With the increasing demands of future information technology for miniaturization and function density, molecular logic gates that are capable of performing Boolean logic operation in response to physical, chemical, and biological inputs are becoming the inspiring trend in forefront of research.1−6 Following the pioneering work of de Silva in 1993,7 different materials such as organic molecules,8−12 nucleic acids,13−15 proteins,16−18 and micro- and nanomaterials19−27 have been successfully demonstrated to construct all 16 fundamental logic gates and higher functions. Although valuable, investigation on the new functional materials for the next-generation molecular logic gates with better applicability still remains a big challenge. Due to its outstanding chelation ability for metal ions via surface amino groups, polyethylenimine (PEI), a water-soluble synthetic cationic polymer, has been extensively exploited for the removal of toxic metal ions, such as cadmium or chromium, from water or blood28,29 and for the development of the watersoluble fluorescent probes toward the detection of metal ions © XXXX American Chemical Society

Received: February 18, 2016 Accepted: March 23, 2016

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DOI: 10.1021/acsami.6b02080 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic representation for the synthesis of functionalized PEIs and their sensing process toward metal ions and anions.

Figure 2. (A) Fluorescence excitation spectrum of R6G (a) and PEIR-H+ (b) and emission spectrum of PEI (c), R6G (d), PEIR (e), and PEIR-H+ (f). (B) Fluorescence response of PEIR-H+ toward various metal ions in aqueous solution. (C) Fluorescence spectra of PEIR-H+ with different concentrations of Cu2+ (0 to 5 mM) upon excitation at 500 nm. Inset: the Stern−Volmer (S−V) plot of PEIR-H+ florescence intensity responding to Cu2+. (D) Fluorescence spectra of PEIR-H+ with different concentrations of Fe3+ (0 to 18 mM) upon excitation at 500 nm. Inset: the S−V plot of PEIR-H+ florescence intensity responding to Fe3+.

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by amino groups in priority. Because protonation can result in the opening of the spirolactam ring and thus form a delocalized xanthenes moiety of the rhodamine group,36−38 it is anticipated that the fluorescence of PEIR should be restored in acidic solution. As expected, aqueous PEIR solution with acidity of pH 3.0 exhibited a strong emission peak centered at 555 nm under the excitation of 525 nm (Figure 2A), and the emission intensity gradually enhanced along with the increase in solution acidity (Figure S1B). Then, the fluorescence responses of PEIR in acidic solution (pH 3.0) toward various metal ions were further investigated. As shown in Figure 2B, surprisingly, the addition of Cu2+ and Fe3+ caused a significant fluorescence quench in PEIR-H+ system, while the other ions demonstrated less or no effect, indicating the fluorescence of the PEIR-H+ system was sensitive to Cu2+ and Fe3+. It is highly possible that the amino groups at PEIR can bind the Cu2+ to form cupric amine complexes which may quench the fluorescence of ringopened amide form of R6G via energy transfer.30−32 It is wellknown that Fe3+ has stronger electron-accepting ability than any other metal ions. Therefore, it is also possible that Fe3+ chelated by the amino groups on PEIR can quench its fluorescence through the electron-transfer mechanism.40−42 To gain further insight into the quenching mechanism, we performed time-correlated single-photon counting (TCSPC) experiments to investigate the energy- and electron-transfer process of PEIR-H+ in the absence and presence of Cu2+ or Fe3+. As shown in Figure S3A and Table S1, the slightly reduced lifetime of PEIR-H+ in the presence of Cu2+ or Fe3+ strongly suggests a static quenching process in which a nonfluorescent complex is formed between the fluorophore and the quencher.53 The static quenching was also validated by the temperature-independent quenching behavior of PEIR-H+ in the presence of Cu2+ or Fe3+ (Figure S4A). The presence of H+ significantly increases the QY of PEIR, and the QY of PEIRH+ is obviously decreased by the addition of Cu2+ or Fe3+ (Table S2), correlating well with fluorescence observations and static quench mechanism. All above results suggest that the amino groups at PEIR can bind the Cu2+ or Fe3+ to form nonfluorescent complexes that are derived from energy- and electron-transfer and thus exhibiting on−off fluorescence behavior. Unavoidably, the nonspecific chelation between amino groups and metal ions resulted in the interference from Ni2+, Co2+, Fe2+, and Cr3+ to some extent. The analytical performance of the PEIR-H+ system for the detection of Cu2+ or Fe3+ was further evaluated. As shown in Figure 2C,D, the fluorescence intensity of PEIR-H+ at 555 nm decreases linearly along with the increase of the concentration of Cu2+ or Fe3+. The fluorescence quenching process of PEIRH+ by Cu2+ or Fe3+ follows the Stern−Volmer (S−V) equation: F0/F = 1 + KSV[Q].43 The S−V plot for Cu2+ shows linearity in the concentration range of 0−2.0 mM, yielding a KSV value of 1.57 × 103 M−1, while Fe3+ demonstrates a wider linearity (0− 6.0 mM) and a lower Ksv value (2.37 × 102 M−1). According to the formula for the detection limit (LOD): LOD = 3.3(SD/ S),33 LOD values for Cu2+ and Fe3+ are calculated to be 21.2 μM and 33.9 μM, respectively. In addition, the Benesi− Hildebrand plot of 1/(F − F0) versus 1/[Mn+] exhibits a straight line for PEIR-H+, suggesting a 1:1 stoichiometry of PEIR-H+ with Cu2+ or Fe3+ (Figure S1C,D).44 The association constant value of log Ka was calculated by dividing the intercept by the slope of the straight line,25 yielding a value of 2.93 and 1.56 for PEIR-H+−Cu2+ and PEIR-H+− Fe3+, respectively.

terminated nanoparticle surfaces via carbodiimide chemistry.33,34 Because molecular logic gates always employ photons, ions, or biomolecules as input and monitor the changes in fluorescent, colorimetric, or electrochemical signals as outputs, it is also possible to design molecular logic gates basing on chemically triggered fluorescence switching of functionalized PEI. Herein we synthesized two new fluorescent probes by covalently grafting PEI with rhodamine 6G (R6G) or 8chloroacetyl-aminoquinoline (CAAQ), and established the functionalized PEI-based fluorescence off-on and on−off probes which response to the metal ions as well as anions in aqueous solution (Figure 1). The new functionalized PEI-based logic systems capable of performing elementary logic operations (YES, NOT, NOR and INHIBIT) and integrative logic operations (OR + INHIBIT), are also implemented by observing the change in the fluorescence with varying the chemical inputs such as protons, metal ions and anions. The rationale for implementing various logic operations relies on the association and dissociation of metal ions (Cu2+, Fe3+ or Zn2+) with functionalized PEIs and their corresponding fluorescence intensity.

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RESULTS AND DISCUSSION PEI has been extensively exploited for the stabilization of nanoscale fluorophore and further development of nanoprobes toward metal-ion detection by taking advantages of the nonspecific chelation between amino groups and metal ions. The specificity can be significantly improved by conjugating PEI with specific recognition elements such as organic molecules, taking advantages of the ease of chemical modification of the abundant primary amine in PEI structure. Rhodamine and quinoline are extensively investigated as platforms for developing specific fluorescent probes toward metal ion detection;33−45 therefore, R6G and CAAQ were covalently conjugated to PEI in this study, respectively, to prepare the functionalized PEI-based probes for metal-ion detection with higher specificity. Rhodamine derivatives have two forms, namely spirolactam and ring-opened amide, which have a remarkable difference in photoluminescence properties. Alternation from spirolactam to ring-opened amide will result in significant fluorescence emission changes, based on which a variety of rhodamine derivatives have been designed as probes for the specific detection of metal ions such as Pb2+,35 Cu2+,36−38 and Fe3+.39 However, most of these rhodamine derivatives typically exhibit poor water solubility, partially limiting their practical applications. This inconvenience can be easily overcome by conjugating them onto hydrophilic polymers.38 Therefore, in this study, R6G was covalently conjugated to branched PEI to synthesize a new type of rhodamine derivatives, PEIR. 1H NMR and FT-IR spectra confirmed the successful conjugation of R6G onto the PEI (Figure S1A and Figure S2). The same as in previous reports in which R6G reacted with hydrazine hydrate,36−38 the obtained PEIR predominately exists in a nonfluorescent spirolactam form. Unexpectedly, upon the addition of various metal ions at pH 7.0, including Hg2+, Ba2+, Mn2+, Pb2+, Li+, Zn2+, Ca2+, Sn2+, Mg2+, Bi2+, Na+, K+, Al3+, Cr3+, Fe2+, Co2+, Ni2+, Fe3+, and Cu2+, the spirolactam ring of PEIR did not seem to open, and no fluorescence change was observed (data not shown). This observation is assumed to be due to the multidentate anchor together with the steric structure existing in PEI, rendering the chelation of metal ions C


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Figure 3. (A) Fluorescence excitation spectrum of PEIQ (a) and PEIQ−Zn2+ (b) and emission spectrum of PEI (c), CAAQ (d), PEIQ (e), and PEIQ−Zn2+ (f). (B) Fluorescence response of PEIQ toward various metal ions in aqueous solution. (C) Fluorescence spectra of PEIQ with different concentration of Zn2+ (0 to 30 μM) upon excitation at 365 nm; Inset: the plot of PEIQ fluorescence intensity responding to Zn2+. (D) Fluorescence response of PEIQ−Zn2+ toward various anions. (E) Fluorescence spectra of PEIQ−Zn2+ with different concentrations of CrO42− (0 to 40 μM) upon excitation at 365 nm. Inset: the S−V plot of PEIQ−Zn2+ fluorescence intensity responding to CrO42−.

Considering the unsatisfactory specificity of PEIR, specific chelation sites for targeted metal ions formed by the amine N atoms from PEI together with other atoms from grated molecules might be more desirable for improving the specificity of functionalized PEI. Due to their strong metal coordination and pH insensitivity, quinoline derivatives have been studied extensively for the development of specific fluorescent probes toward metal ions.33,34,45−47 Therefore, in this study, PEI was grafted with CAAQ to obtain a new type of quinoline derivatives, PEIQ, to overcome the inconvenience of most

quinoline-based probes such as poor water solubility and bad cell permeability. As confirmed by 1H NMR and FT-IR spectra, CAAQ was successfully conjugated onto PEI to form PEIQ (Figure S5A and Figure S2). As shown in Figure 3A, a broad and weak emission centered at 450 nm was depicted in fluorescence spectra of PEIQ (curve e of Figure 3A). The response of PEIQ toward several metal ions (Al3+, Ba2+, Bi2+, Ca2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, Sn2+, and Zn2+) is examined. As anticipated, dramatic enhancement of fluoresD


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Figure 4. Schematic representation of logic gates obtained for PEIR−H+−Cu2+−Fe3+ and PEIQ−Zn2+−CrO42− systems.

linearly increases in the concentration range from 0 to 6.0 μM. The quantification of Zn2+ by PEIQ could be achieved with a correlation coefficient of 0.999, and its LOD is about 38.1 nM. Metal ions play many important roles in chemistry and biological fields. For example, copper ions are essential to the processes of bone formation, cellular respiration, and connective tissue development, and they also serve as enzyme cofactors. A series of neurodegenerative diseases, such as Wilson’s disease, Alzheimer’s disease, Menke’s disease, and Parkinson’s disease are associated with copper-ion imbalance.48 Ferric iron acts as an oxygen transporter in heme and a catalytic cofactor in many enzymatic reactions. The deficiency of Fe3+ may lead to anemia and permanent loss of motor skills, and the excess amount of iron may result in organ dysfunction and neural disorders and is attributed to several types of cancers.49 The essentiality of zinc has been recognized in many critical biological processes such as gene transcription, neurotransmission, metalloenzyme function, and cell apoptosis. Growing evidence suggests that the disruption of Zn2+ homeostasis leads to severe pathological consequences such as neurological diseases, including Alzheimer’s disease, infantile diarrhea, epilepsy, and cerebral ischemia.50 Therefore, to better understand the physiological roles of metal ions, it is of great significance to develop facile methods for their sensitive and specific detection in biological media. In addition to the examination of PEIQ response toward metal ions, the response of the PEIQ−Zn2+ system toward anions, including CrO42−, HPO4−, B4O72−, OH−, HCO3−, H2PO4−, SO32−, CO32−, NO3−, SO42−, Br−, Cl−, F−, and NO2−, was also examined. Interestingly, the enhanced fluorescence of PEIQ-Zn2+ system was quenched upon the addition of anions. As demonstrated in Figure 3D, the addition of CrO42− induced the significant fluorescence quench in the PEIQ−Zn2+ system, possibly due to the much stronger affinity between Zn2+ and CrO42−. In contrast, other anions demonstrated less or no influence on the fluorescence of the PEIQ−Zn2+ system, displaying good specificity toward CrO42−. A dynamic

cence emission with a sharpened and red-shifted peak centered at 510 nm was observed only in the presence of Zn2+ (curve f of Figure 3A,B). PEIQ is most likely to coordinate Zn2+ via two amine N atoms from PEI, and quinoline N, amide N atoms from CAAQ to induce chelation-enhanced fluorescence,45−47 as evidenced by the TCSPC measurements that reveal a significantly increased lifetime of PEIQ in the presence of Zn2+ (Figure S3B and Table S1). The QY of PEIQ increases 3fold after the chelation with Zn2+ (Table S3), correlating well with fluorescence observations and TCSPC measurements. The radiative rate constant (kr) of PEIQ and PEIQ−Zn2+ are calculated to be 7.13 × 105 and 1.4 × 106 (Table S4),54,55 suggesting a strong chelation between PEIQ and Zn2+, as evidenced by a 20-fold enhancement in kr value. As shown in Figure S4B, it is obvious that the lower temperature is conducive to the fluorescence of PEIQ−Zn2+, presenting another evidence for chelation-enhanced fluorescence mechanism. Because PEI provides another two binding sites for coordination, the higher specificity of PEIQ toward Zn2+ over other metal ions is expected. According to the Benesi− Hildebrand method, the stoichiometry for interaction of PEIQ with Zn2+ was determined to be 1:1, and the association constant value of log Ka was calculated to be 4.99 (Figure S5C). It is obvious that the affinity between PEIQ and Zn2+ is substantially stronger than that between PEIR and Cu2+ or Fe3+, possibly due to multiple coordination atoms both from PEI and grated CAAQ. Although quinoline is insensitive to acidity, low pH values may lead to the protonation of the imino groups of PEIQ, thus weakening its coordination ability toward Zn2+.33,54 As shown in Figure S5B, the alkaline condition favored the detection of Zn2+ using PEIQ, while only negligible fluorescence emission could be observed in the pH range of 1.0−6.0 for the solution containing both PEIQ and Zn2+. The analytical performance of PEIQ for the detection of Zn2+ was further evaluated. As shown in Figure 3C, the fluorescence intensity of PEIQ at 510 nm under the excitation of 365 nm E


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Figure 5. NOR gate. (A) Normalized fluorescent spectra of PEIR−H+-based NOR gate with different input combinations upon excitation at 500 nm. Inset: relative fluorescence intensities at 555 nm for the NOR gate with different input combinations. The symbol and the truth table of the NOR gate. (B) Normalized fluorescent spectra of PEIQ−Zn2+-based NOR gate with different input combinations upon excitation at 365 nm. Inset: relative fluorescence intensities at 510 nm for the NOR gate with different input combinations. The symbol and the truth table of the NOR gate.

quenching process occurs when CrO42− reacts with PEIQ− Zn2+, validated by the significantly reduced lifetime of PEIQ− Zn2+ in the presence of CrO42− (Figure S3B and Table S1). The QY of PEIQ−Zn2+ is obviously decreased by the addition of CrO42− (Table S3), correlating well with fluorescence observations and dynamic quench process. Due to the weakened collision between CrO42− and PEIQ−Zn2+, the quenching efficiency decreased along with the temperature decreased (Figure S4B), further verifying the dynamic quenching process. The fluorescence of PEIQ−Zn2+ was gradually quenched along with increasing concentration of CrO42− ion (Figure 3E). Approximately 80% of the fluorescence at 510 nm was quenched by the addition of 40 μM CrO42−. The LOD of the PEIQ−Zn2+ system for CrO42− was calculated to be 1.08 μM. CrO42− is reported to be highly toxic to animals and humans by causing damages in cardiovascular system, kidney, liver, and hematogenic function,51 and the detection of CrO42− with high sensitivity and specificity in biological media is thus highly desired. Inspired by the observations in the sensing capabilities of functionalized PEI toward metal ions and anions, we develop a fluorometric logic system capable of performing individual logic operations such as YES, NOT, NOR, and INHIBIT (Figure 4), and integrative logic operations (OR + INHIBIT). In logic operations, functionalized PEIs (PEIR or PEIQ) served as gates, while proton (H+), metal ions (Cu2+, Fe3+, or Zn2+) and anion (CrO42−) were used as chemical inputs. The presence and absence of these inputs were coded with Boolean logic

functions as 1 and 0, respectively. The output value was defined as 1 when the relative fluorescence intensity was higher than 0.3, whereas the output value was defined as 0 when the relative intensity was lower than 0.3. As demonstrated in Figures 2 and 3, H+ and Zn2+ can, respectively, trigger the fluorescence turn-on response of PEIR and PEIQ, while Cu2+ and CrO42− can, respectively, trigger the fluorescence turn-off response of PEIR-H+ and PEIQ-Zn2+. These results are consistent with the Boolean logic operations YES and NOT which are driven by a single input. Therefore, the simplest logic gate YES, whose output signal is the same as the input signal, was achieved by presenting H+ as the only input to PEIR or Zn2+ as the only input to PEIQ. Similarly, the logic gate NOT, with an output signal opposed to the input signal, was constructed by presenting Cu2+ as the only input to the PEIR−H+ system or CrO42− as the only input to the PEIQ−Zn2+ system (Figure 4). To be a more useful system, a logic gate should be multipleconfigurable. As demonstrated in Figures S2B and 3D, either H+ or CrO42− can trigger the fluorescence turn-off response of PEIQ−Zn2+. Similarly, either Cu2+ or Fe3+ can trigger the fluorescence turn-off response of PEIQ−H+ (Figure 2C,D). These results are consistent with the NOR logic operation, which produces an output of 1 only in the absence of any inputs. As illustrated in Figure 5A, PEIR−H+ system only exhibited strong fluorescence in the absence of two inputs Cu2+ and Fe3+, but obvious fluorescence quench occurred for all the other input combinations, correlating well with the proper execution of NOR logic operation. Similarly, an identical NOR F


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Figure 6. INHIBIT gate. (A) Normalized fluorescent spectra of PEIR-based INHIBIT gate with different input combinations upon excitation at 500 nm. Inset: relative fluorescence intensities at 555 nm for the INHIBIT gate with different input combinations. The symbol and the truth table of the INHIBIT gate. (B) Normalized fluorescent spectra of PEIQ-based INHIBIT gate with different input combinations upon excitation at 365 nm. Inset: relative fluorescence intensities at 510 nm for the INHIBIT gate with different input combinations. The symbol and the truth table of the INHIBIT gate.

logic operation can be implemented by employing PEIQ−Zn2+ as the gate while H+ and CrO42− function as double inputs (Figure 5B). A binary INHIBIT logic operation, which is represented by the situation in which the output is 1 only if one particular input is present and the other is absent, can be implemented as well using PEIQ as gate while Zn2+ and CrO42− as double inputs (Figure 6B). The INHIBIT logic operation relies on the principle that CrO42− can interact with Zn2+ to diminish the interactions between Zn2+ and PEIQ (Figure 3E). PEIQ alone (input = 0,0) is nonfluorescent (output = 0), but the addition of Zn2+ only (input = 1,0) triggered the fluorescence turn-on response of PEIQ (output = 1). CrO42− alone (input = 0,1) does not impact the PEIQ fluorescence properties (output = 0) but can trigger the fluorescence turn-off response of PEIQ− Zn2+. Therefore, in the presence of both Zn2+ and CrO42− (input = 1,1), the solution remains nonfluorescent (output = 0). On the basis of the fact that H+ can trigger the fluorescence turn-on response of PEIR while Cu2+ can quench the fluorescence of the PEIR−H+ system, the INHIBIT logic operation can also be implemented by employing PEIR as the gate while Cu2+ and H+ functioned as double inputs (Figure 6A). These results correlate well with the proper execution of INHIBIT logic operation. The challenge still exists in the construction of sophisticate molecular devices with multiple inputs. The complexity of logic operations would be increased when more inputs are integrated, and versatile higher functions could be implemented

by the concatenation of different logic gates. However, most of the reported logic systems are the lack of integration or concatenation between the logic gates. In this study, we introduced the third input to the functionalized PEI-based logic systems to construct more complicated molecular logic circuits. As demonstrated in Figure 2, H+ can trigger the fluorescence turn-on response of PEIR, while either Cu2+ or Fe3+ can trigger the fluorescence turn-off response of PEIR−H+. It is thus possible to construct an integrative logic system by employing PEIR as the gate and its fluorescence signal as output. This logic gate has three inputs (H+, Cu2+, and Fe3+) that generate eight situations in total. When H+ is absent from the solution (input = 0,0,0; 0,1,0; 0,0,1; and 0,1,1), PEIR remains nonfluorescent (output = 0) because only H+ can trigger the fluorescence turn-on response of PEIR (Figure 7A). In the case that H+ is present (input = 1,0,0; 1,1,0; 1,0,1; and 1,1,1), due to the quenching effect of Cu2+ or Fe3+ toward the fluorescence of PEIR−H+, the presence of H+ and either of two metal ions (input = 1,1,0; 1,0,1; and 1,1,1) induce the fluorescence turn-off response (output = 0). On the basis of the fact that Zn2+ can trigger the fluorescence turn-on response of PEIQ while either H+ or CrO42− can trigger the fluorescence turn-off response of PEIQ−Zn2+, a similar integrative logic gate can also be constructed by employing PEIQ as the gate while Zn2+, H+, and CrO42− function as inputs (Figure 7B). Therefore, by taking advantages of the dual response of functionalized PEIbased systems (PEIR−H+ or PEIQ−Zn2+) to chemical inputs, three-input logic operations that realize the functionality of OR G


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Figure 7. Circuit for the three-input logic gate OR-INHIBIT gate. (A) Normalized fluorescent spectra of PEIR-based OR-INHIBIT gate with different input combinations upon excitation at 500 nm. Inset: relative fluorescence intensities at 555 nm for the OR-INHIBIT gate with different input combinations. The symbol and the truth table of the OR-INHIBIT gate. (B) Normalized fluorescent spectra of the PEIQ-based OR-INHIBIT gate with different input combinations upon excitation at 365 nm. Inset: relative fluorescence intensities at 510 nm for the OR-INHIBIT gate with different input combinations. The symbol and the truth table of the OR-INHIBIT gate.

implementing various logic operations relies on the association and dissociation of ions with functionalized PEIs and their corresponding fluorescence intensity. We believe these highly sensitive and responsive functionalized PEIs are promising elements for the constructions of next-generation molecular devices with better applicability for biomedical research.

plus INHIBIT have been implemented successfully (Figure 7), demonstrating that the information have been successfully transferred from one logic gate to another by chemical induction. Recently, besides attempts to design sophisticated molecular devices using multiple inputs or modular coupling of the computing elements, several systems have been demonstrated for the practical biomedical applications such as intelligent diagnostics,11,56 multiplexed detection,57 bioimaging,6 regulation of protein folding and activity,17,58 and drug delivery,26,59,60 in which outputs manifested as effective functions controlled by particular inputs. Due to their high sensitivity and fast response, fluorescent materials, such as functionalized PEIs, group 9 organometallic compounds,61−63 carbon nanodots,22,25,64 and silver and gold nanoclusters,65,66 are considered to be the most promising elements for the design of next-generation molecular logic devices with better applicability for biomedical research.

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EXPERIMENTAL SECTION

Materials. The metal ion solutions were prepared from NaCl, KCl, MgCl2, CaCl2, FeCl3, Pb(NO3)2, CoCl2, NiCl2, ZnCl2, HgCl2, CrCl3, and CuSO4 in distilled water with a concentration of 10.0 or 1.0 mM. The anion solutions were prepared from Na2SO3, NaHCO3, NaNO2, NaOH, NaNO3, NaCl, Na2HPO4, Na2CO3, KBr, Na2SO4, NaF, NaH2PO2, Na2CrO4, and Na2B4O7 in distilled water with a concentration of 20.0 mM. Triethylamine, dichloronmethane, ethanol, K2CO3, and acetonitrile were purchased from a domestic analytical reagents brand. Rhodamine 6G (R6G) was purchased from SigmaAldrich. 8-Aminoquinoline (98%) and polyethylenimine (M.W. 1800, 99%) were purchased from Aladdin (Shanghai, China). Chloroacetyl chloride was purchased from J & K chemical. Instruments. 1H NMR spectra were recorded with a Varian Mercury-400 spectrometer with Me4Si as the internal standard. Fluorescent spectra and fluorescence lifetimes were collected on a Fluoromax-4P spectrometer (Horiba) and a Fluorolog-3 spectrometer (Horiba), respectively.

CONCLUSIONS

In summary, elementary logic operations (YES, NOT, NOR, and INHIBIT) and integrative logic operations (OR + INHIBIT) have been constructed by employing PEIR and PEIQ as gates while protons, metal ions (Cu2+, Fe3+, or Zn2+), and anions (CrO42−) as chemical inputs. The rationale for H



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Synthesis of PEIR. A mixture of R6G (0.02 g) and PEI (MW = 1800) (1.0 g) in 15 mL of EtOH was heated to reflux temperature for 24 h. After cooling, the solution was dialyzed against deionized water using a dialysis membrane with MWCO of 1kD for 3 days to remove excess R6G. Finally, the solvent was evaporated under reduced pressure, and the obtained residue was stored at 4 °C before further usage. Synthesis of PEIQ. 8-chloroacetylaminoquinoline (CAAQ) was synthesized according to a previously reported method with a slight modification.52 In brief, 288 mg of 8-aminoquinoline (2 mM) and 202 mg (2.1 mM) of N(Et)3 were mixed in 10 mL of CH2Cl2 for 20 min at 0 °C, followed by the addition of 246 mg (2.2 mM) of chloroacetyl chloride. The mixture was allowed to react overnight at room temperature. Then, the crude product was obtained by evaporating the solvent in vacuum and further purified by column chromatography (silica gel, PE/EA at 3:1) to give a pale white solid. A mixture of PEI (MW = 1800) (1.98 g, 1.10 mmol), K2CO3 (150.0 mg, 1.1 mmol) was added to 20 mL of CH3CN solution containing CAAQ (220 mg, 1 mmol), and the mixed solution was allowed to reflux for 8 h under N2. Then, the crude product was obtained by evaporating the solvent in vacuum and purified by coprecipitation with ether to give a yellow oil. Characterizations of PEIR and PEIQ. As shown in Figure S1A, typical resonance peaks between 6.0 and 8.0 ppm originated from the heteroaromatic rings are displayed in the 1H NMR spectrum of R6G. Although the peaks of PEI only appeared at 2.6 and 4.85 ppm, which were far away from those of R6G. From the comparison, it could be observed that the PEIR contained the characteristic peaks of both PEI and R6G, indicating the successful grafting PEI with R6G. As showed in the 1HNMR spectrum, CAAQ displayed typical resonance peaks between 7.0−11.0 ppm, while the peaks of PEI appeared at 2.6 and 4.85 ppm, which were far away from those of CAAQ (Figure S3A). As expected, PEIQ have the characteristic peaks of both PEI and CAAQ, indicating the successful grafting PEI with the quinoline derivative. In the FT-IR spectrum of PEIR and PEIQ (Figure S2), not only the peaks originated from the skeletal vibration of the heteroaromatic rings of R6G and CAAQ but also the vibration peaks of −NH2 groups from PEI were presented. In addition, more peaks were found in PEIR and PEIQ at around 1600−1670 cm−1, which can be attributed to the bond stretching of CC, CN, or CO in comparison to PEI (Figure S2), indicating the successful conjugation of R6G and CAAQ to the PEI, respectively. General Procedure for Sensing of Metal Ions and Anions. Functionalized PEI-based fluorescent probes (PEIR and PEIQ), metal ions, and anions stock solutions were prepared in water. The fluorescence titration measurements were carried out by adding small volumes of metal ions or anions to the solution containing the probes. After the addition of metal ion or anion, the mixtures were mixed well and kept for 5 min before measurement.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: (86)-20-39342380; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Pearl River S&T Nova Program of Guangzhou (2013J2200053), and Tip-Top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2014TQ01R417) is gratefully acknowledged.

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REFERENCES

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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/acsami.6b02080. Figures showing 1HNMR and FT-IR spectra, fluorescence decay profiles of the functionalized PEIs, the sensing capabilities of the functionalized PEIs under different temperatures, plot of fluorescence intensity of the functionalized PEIs as a function of the pH values, and a Benesi−Hildebrand plot of 1/(F0 − F) versus 1/ [Mn+] for the functionalized PEIs. Tables showing fluorescence time and quantum yield data for the functionalized PEIs and calculation of kr and knr for PEIR and PEIQ. (PDF) I


Research Article


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K


Design of Multiple Logic Gates Based on Chemically Triggered

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