Î±-Cyclodextrin Functionalized Carbon Dots: Pronounced

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#-Cyclodextrin Functionalized Carbon Dots: Pronounced Photoinduced Electron Transfer by Aggregated Nanostructures Somen Mondal, and Pradipta Purkayastha J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03145 • Publication Date (Web): 10 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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The Journal of Physical Chemistry

α-Cyclodextrin Functionalized Carbon Dots: Pronounced Photoinduced Electron Transfer by Aggregated Nanostructures Somen Mondal, and Pradipta Purkayastha* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, WB 741246, India.

ACS Paragon Plus Environment

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ABSTRACT: In recent past assembly of α-cyclodextrin (α-CD) functionalized carbon dots (αCD-CDots) has been used in molecular recognition. These assemblies are effectively used for optical sensing by the help of electron transfer mechanism. Photoinduced electron transfer (PET) between the CDots and surface-encapsulated derivatives of methyl viologen (MV2+) is used in the present work to follow the effect of the formed aggregated nanostructures. Formation of the nanoaggregates was confirmed by fluorescence anisotropy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). These aggregated nanostructures are found to reinforce the electron transfer dynamics between CDots and MV2+. PET has been confirmed from steady-state and time-resolved fluorescence along with ultrafast transient absorption measurements. The present work elaborates kinetic details of PET in the formed nanotubular aggregates. The reported device may be prospective towards development of light energy conversion devices.


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INTRODUCTION Fluorescence resonance energy transfer (FRET) and electron transfer (ET) are useful tools to detect DNA hybridization, for metal ions recognition, and to analyze biocatalytic transformations.1-3 Efficient electron transfer (ET) is a major focus of research toward biosensing, water purification, solar energy conversion, molecular electronics,4-6 etc. Recently, Freeman at el. has investigated the FRET or a direct electron transfer quenching based competitive assay using β-cyclodextrin-modified CdSe/ZnS QDs as sensors and chiroselective sensors.7 Jia at el. described a direct enzyme activity sensing nanobiosensor platform based on βCD-functionalized CdTe QDs.8 In these contexts, fluorescent carbon dots (CDots) are proven to be superior to semiconductor QDs as they possess good physicochemical as well as photochemical stability, optical brightness, and are non-blinking and non-toxic.9 We have recently reported on the usability of CDots as a medium for ET.10-12 In ET phenomenon, the injection and recombination rates depend on the strength of coupling between the electron donor and the acceptor. ET aided by excitation of species with the help of light energy is called photoinduced electron transfer (PET). It is known that the efficiency of PET depends on the relative distance between donor−acceptor interfaces. Therefore, nanostructure aggregation may prove to be promising for fabricating photoconversion devices in future. Such devices come under the domain of supramolecular chemistry and provide excellent level of control over the distance between the photoactive centres.13-15 Supramolecular chemistry is based on molecular recognition principally through host–guest chemistry. Among many prospective hosts, cyclodextrins (CDs) occupy a high position because of several advantages including biological compatibility and aqueous solubility.16-18 Among the CDs, the smallest cavity size is provided by the α-variants, which are cyclic oligosaccharides


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with six glucose units linked at the α-1,4 positions to form a truncated cone-like structure.16-18 Non-covalent interactions, such as, hydrogen and ionic bonding, hydrophobic and van der Waals interactions are supposed to be the driving forces in the formation of CD supramolecular aggregates. CDs can form inclusion complexes with hydrophobic guest molecules in aqueous solution predominantly due to hydrophobic interactions. With an aim to create nanoparticulate array of CDots to enhance PET, herein, we have modified the CDots with boronic acid and linked α-CD units to them via the secondary vicinal hydroxyl groups of the sugar units in the CDs. The constituted array was used to study PET between the CDots and different derivatives of methyl viologens that are encapsulated within the α-CD cavities. It is known that CDots can act as electron donor as well as acceptor and methyl viologen is a very good electron acceptor.10-12 The structural difference of the methyl viologen derivatives determines the nature of the nanoparticulate aggregates formed by the α-CD-CDot units. Effect on PET in these aggregates was monitored by observing the quenching of CDot fluorescence. The derivatives of methyl viologen used are 1,1ʹ-Diheptyl-4,4ʹ-bipyridinium dibromide (DHMV2+), 1-Heptyl-4-(4-pyridyl) pyridinium bromide (HMV+), and the parent methyl viologen (MV2+) (Scheme 1) for controlling PET with the CDots. Steady-state and timeresolved fluorescence coupled with ultrafast transient absorption spectroscopy were used to show that PET gets substantially pronounced in this suprastructural system.

N

N

N

N

N

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HMV+


N

MV2+

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Scheme 1. The derivatives of methyl viologen (MV2+) used in the present study.

EXPERIMENTAL SECTION

Materials. Citric acid, ethylenediamine, 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride

(EDC.HCl),

N-hydroxysulfosuccinimide

sodium

salt

(Sulfo-NHS),

3-

aminophenylboronic acid, α-Cyclodextrins, 1,1´-Diheptyl-4,4´-bipyridinium dibromide, 1Heptyl-4-(4-pyridyl)pyridinium bromide and Methyl viologen were purchased from SigmaAldrich and used as received. Triple-distilled water was used to prepare the experimental solutions. Preparation of Carbon Dots (CDots). Citric acid (1.0 g) and ethylenediamine (0.149 mL) having mole ratio of NH2/COOH = 0.20 in the precursors were added to 10 mL of water, which was sonicated for 6 min. to get a clear solution. The solution was then put into a 750 W microwave oven operating at 80 °C and incubated for 2 min. Finally, the CDots were poured into ethanol, and the precipitate was separated by centrifugation.19 Preparation of p-Aminophenylboronic acid capped CDots. The carboxyl coated CNPs (0.2 g) were dissolved in 5 ml water and sonicated for 15 min. EDC.HCl (19.2 mg, 0.1 mmol) and Sulfo-NHS (21.7 mg, 0.1 mmol) were added into the CDots solution under N2 atmosphere and stirred for 1 h at room temperature. This is followed by addition of p-aminophenylboronic acid (0.1 mmol) to the mixture and stirred for 48 h. Finally, the excess p-aminophenylboronic acid was removed by two successive precipitation steps, and the purified particles were dissolved in phosphate buffer, 0.1 M, pH 10.4. Preparation of α-CD-capped CDots. 3-aminophenylboronic acid-functionalized CDots was added to 1 ml of 80 mM α-CD, dissolved in phosphate buffer, and the mixture was shaken for 12


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h. The excess of α-CD was removed by two successive precipitation steps. The α-CD functionalized CDots precipitate in methanol and excess α-CDs remain in solution. Methods. The absorption spectra were recorded using a Cary 300 Bio UV-Vis spectrophotometer from Agilent. Fluorescence measurements were done using a QM-40 spectrofluorimeter procured from PTI. The fluorescence lifetimes were measured by the method of time-correlated single-photon counting (TCSPC) using a picoseconds spectrofluorimeter from Horiba Jobin Yvon IBH equipped with a FluoroHub single photon counting controller, Fluoro3PS precision photomultiplier power supply and FC-MCP-50SC MCP-PMT detection unit. 402 nm laser head was used as the excitation source. The excitation pulse duration is 1 µsec, repetition rate is 1MHz and time resolution is >100 ps. The FTIR spectra were recorded using a Perkin–Elmer Spectrum RX1 spectrophotometer. A KBr pellet was made by taking roughly 2 mg of the sample with 20 mg of KBr. The dynamic light scattering (DLS) measurements were done using a Malvern Zetasizer Nano equipped with a 4.0 mW HeNe laser operating at λ = 633 nm. The sample was measured in an aqueous system at room temperature with a scattering angle of 173o. The size distribution was calculated by Nano software using a non-negative least square analysis (NNLS). The atomic force microscopy (AFM) was performed using an NT-MDT NTEGRA instrument procured from NTMDT, CA, USA. The ultrafast transient absorption measurements were performed by using a femtosecond pump−probe setup, which consisted of a mode-locked Ti-sapphire oscillator (Spitfire, Spectra Physics) that served as the seed laser for the amplifier, generating femtosecond pulses (fwhm < 100 fs, ∼2.5 W at 80 MHz).

RESULTS AND DISCUSSION


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The CDots were prepared by a facile method from citric acid source by microwave treatment following a reported protocol.19 These CDots were derivatized with boronic acid ligands using 1(3-Dimethylaminopropyl)-3-ethylcarbodiimide

hydrochloride

(EDC.HCl),

N-

hydroxysulfosuccinimide sodium salt (Sulfo-NHS) and 3-aminophenylboronic acid as coupling agent. The α-CD units were conjugated with boronic acid using the hydroxyl groups of the sugar units of CD at pH 10.4. The absorption spectrum exhibits a broad shoulder at ~260 nm without noticeable structural features and a weak band at ~360 nm as shown in Figure 1A. According to previous reports, the former peak is attributed to π-π* transitions in the aromatic C=C bonds and the latter to the n-π* transitions in the CDots.20-22 The surface functionalities of the CDots were determined from the FTIR spectrum. The peaks at 3412 and 1588 cm-1 can be assigned to –OH and –C=O stretching. Bands at 1478, 1400, 1298 cm-1 were assigned to the –N-H, -C-N and -CO, respectively. The fourier transform infrared spectroscopy (FTIR) data suggest existence of carboxyl groups (probably at the surface) and the C=C groups constitute the core.19 A strong blue fluorescence is observed with a maximum at 450 nm which is independent of excitation wavelength between 300-420 nm. The invariance in the emission peak position with varying excitation wavelength reflects relatively uniform size distribution of the synthesized CDots. The average hydrodynamic diameter of the CDots, as obtained from DLS, is in the range of 3.5 nm (Figure 1B).


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(B)

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Figure 1. Characterization of CDots: (A) absorption and emission spectra and (B) DLS histogram. 3

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(B)

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Figure 2. Characterization of α-CD-CDots: (A) comparison of absorption spectra, and (B) FTIR spectra. The α-CD-CDots were characterized by UV, FTIR (Figure 2) and 1H NMR (Figure S1) spectroscopy. The absorption spectrum of α-CD-CDots, as shown in Figure 2A, exhibits a peak at 275 nm and a shoulder at 360 nm. The red shift of the 260 nm band of the bare CDots indicates adduct formation with α-CD. Figure 2B shows the FTIR spectra for the ethylenediamine functionalized CDots and α-CD-functionalized CDots. The α-CD-CDots were


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designed to attach molecular hosts on CDots that can shelter potential electron acceptors. This nanodevice, in turn, may provide stable platform for the donor and acceptor to facilitate PET.

0 mM

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2.0 1.5 1.0

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Figure 3. Fluorescence spectra of α-CD-CDots in absence and presence of different concentrations of (A) DHMV2+, (B) HMV+, and (C) MV2+; (D) shows the relative quenching plots for the species in terms of Stern-Volmer equation. It is observed that addition of MV2+ and its derivatives quench the fluorescence of the α-CDCDots to different degrees (Figure 3A-C) that can presumably be due to ET. MV2+ is known to be a good electron acceptor and CDots have excellent electron donating as well as accepting properties.10-12,23 The extent of fluorescence quenching is determined by using Stern-Volmer


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equation (Figure 3D). The Stern-Volmer equation is given as I0/I = 1+KSV[Q], where the I0 and I are the fluorescence intensities of CDots in absence and presence of quencher, respectively, KSV is the Stern-Volmer constant and [Q] is the effective concentration of the quencher. The plots of the relative change in fluorescence intensities (I0/I) show that quenching of donor fluorescence is much higher in case of DHMV2+ compared to that by HMV+ and MV2+. Such behavior could be due to variety of supramolecular aggregation of the α-CD-CDots due to the different derivatives of MV2+. In addition, it should be noted that the plot is non-linear in case of DHMV2+. The nonlinearity in Stern-Volmer plot happens due to occurrence of both static and dynamic quenching. In case of DHMV2+, ground state complex formation is more effective than the other two derivatives of methyl viologen (see Supporting Information). 2+

HDMV 2+ MV 2+ DMV

0.30 0.25

Anisotropy

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0.20 0.15 0.10 0.05 0

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Quencher (mM)

Figure 4. Change in steady state fluorescence anisotropy of the α-CD-CDots on interaction with different derivatives of MV2+. Change in fluorescence anisotropy of the α-CD-CDots due to interaction with the MV2+ derivatives indicates that there is a considerable motional restriction on the fluorophore due to addition of DHMV2+ (Figure 4). In this case the variations in supramolecular aggregation, if any, could be brought in by the presence of the hydrophobic tails on the MV2+ derivatives that are


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prone to get encapsulated inside the α-CD cavities on the CDots. Thus, the nature of existence of the hydrophobic chains in DHMV2+ and HMV+ will influence the formation of the aggregates.

Scheme 2. Representative cartoon showing formation of the structural morphologies of the nanotubular aggregates in presence of DMHV2+. Scheme 2 shows probable structural motifs of aggregation of the nanosystems around the αCD-CDots. Formation of nanotubular aggregates with DHMV2+ and spherical aggregates with HMV+ and MV2+ is expected. The presumption towards nanotubular aggregation with DHMV2+ is stemmed from a mathematical model that shows cylindrical arrangements by spheres happening through ‘‘parastichies’’ depending on the spherical contacts and dense sphere packings.24 Such type of aggregations could be found in various biological systems, such as actin, salmonella, tobacco mosaic virus, etc.25 Thus, the spherical aggregates of α-CD-CDots and DHMV2+ could tend to form nanotubular aggregates, which is a natural tendency as discussed above. In HMV+ and MV2+ there is either one or no hydrophobic tail which prevents the formation of spherical aggregates (as in DHMV2+) that, in turn, could lead to the cylindrical


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structures. Figure 5 shows atomic force microscopy (AFM) and scanning electron microscopy (SEM) images that establishes our presumptions. (A)

Figure 5. The upper and the lower panels show AFM (scale bar = 100 nm) and SEM images (scale bar = 200 nm) for the α-CD-CDots in presence of (A) DHMV2+, (B) HMV+ and (C) MV2+ respectively. The PET dynamics were measured by time-resolved fluorescence decay of the α-CD-CDots in presence of DHMV2+, HMV+ and MV2+. Multi-component fit to the excited state lifetime decay data of the α-CD-CDots suggests presence of multiple radiative species in the samples. We considered the average lifetimes of the species. The average lifetimes () of the α-CD-CDots in presence of the different derivatives of MV2+ were calculated using the equation: = (a1τ1+ a2τ2)/(a1+a2), where τ1 and τ2 are the fluorescence lifetimes and a1 and a2 are their relative amplitudes. It is observed that decay becomes faster as the concentration of the derivatives of MV2+ is increased as shown in Table S1. The highest degree of reduction in resulted in


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presence of DMHV2+. HMV+ and MV2+ behave quite similarly, which is consistent to the proposed aggregation motifs of the different α-CD-CDots. Conclusive proof towards occurrence of PET is obtained from ultrafast transient absorption of the α-CD-CDots at different delay times on excitation at 350 nm in absence and presence of the different derivatives of MV2+ as shown in Figure 6. Figure 6A shows traces for the excited-state absorption (ESA) at around 420 nm and stimulated emission (SE) between 430-500 nm. A growth in the α-CD-CDot signal is observed when the 420 nm absorption is monitored after irradiation with pulsed laser within the first few picoseconds (Figure 6 B). Generally, formation of special “molecule-like” states, such as, carboxyl, carbonyl and hydroxyl group in CDots, are responsible for the ESA.26-28 It can be clearly seen that, in this case, kinetics of PET became faster on addition of the different derivatives of MV2+ indicating ready transfer of the surface electron to the acceptor component.

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0.6

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Figure 6. Transient absorption spectra recorded for (A) α-CD-CDots; and (B) kinetic traces at 420 nm of α-CD-CDots in absence (black) and presence of DHMV2+ (red) in aqueous medium at various delay times after excitation using a 350 nm pulse.


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Figure 7. (A) Transient absorption spectra of the α-CD-CDots in the presence of DHMV2+ and (B-D) kinetic traces at 600 nm for PET dynamics in the α-CD-CDots in the presence of DHMV2+, HMV+, and MV2+, respectively in aqueous medium at various delay times after 350 nm excitation. The transient absorption spectra recorded at different delay times following 350 nm laser pulse excitation of the α-CD-CDots in the presence of DHMV2+ at a DHMV2+/CDots ratio of 10:1 are shown in Figure 7A. Similar spectra for HMV+ and MV2+ are provided in Figure S3. The excited state absorption (ESA) bands at 393 nm and 605 nm confirm formation of the radical cation. ET from DHMV2+ to α-CD-Cdots is thus confirmed.29 It can further be verified by the change in lifetime when monitored at 605 nm (Table 1) that shows fast PET in these systems occurring in ∼15 ps. The change in ESA, as shown in Figure 7, can be analyzed by fitting the data with bi-


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exponential fitting routine, where the parameters a1 and a2 are the relative amplitudes of each lifetime component and τ1 and τ2 are the fast and slow components of the excited-state lifetimes, respectively, as provided in Table S1. The electron transfer process is faster in the α-CDCDots/DHMV2+ system than the other derivatives of MV2+ principally because of the formation of the tubular aggregates. The rate constant of PET is defined as the reciprocal of the average lifetime. The calculated rate constants from the traces are 1.04 ×1011 s-1, 0.68 × 1011 s-1 and 0.76× 1011 s-1 for DHMV2+, HMV+ and MV2+ at 600 nm, respectively (Figure 7B-D).

Table 1. Kinetic Parameters of CDots in Excited-State Absorption at 605 nm Quencher τ1 (ps) τ2 (ps) a1 (%) a2 (%) 2+ DHMV 2.22 14.30 60.9 39.1 9.57 HMV+ 1.11 17.60 82.4 17.6 14.7 MV2+ 1.11 35.29 34.8 65.2 13.0

CONCLUSIONS Herein, we report a host-guest approach to build a nanoscale assembly of α-CD-CDots and different derivatives of MV2+ as electron acceptors. It is observed that PET efficiency is highest in presence of DHMV2+ compared to the other derivatives of MV2+ because of formation of nanotubular aggregates. Time resolved fluorescence decay and transient absorption spectroscopy provided conclusive insight for the electron transfer processes at different time scales. The rate electron transfer is found to be 1.5 fold faster in α-CD-CDots and DHMV2+ aggregates than the other two derivatives of MV2+. We believe that this report on PET in self-accumulated nanotubular aggregates in solution will provide a broad understanding of reaction pathways and kinetic details of α-CD-CDot based host-guest donor-acceptor systems and will be important towards applications in light energy conversion.


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ASSOCIATED CONTENT Supporting Information. Characterization of the nanomaterials, normal absorption and transient absorption spectra of α-CD-CDots in absence and presence of different concentrations of the additives, and table with time-resolved fluorescence data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Telephone: +91-9831635082 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The work is supported by DST SERB through the grant number EMR/2015/000950. SM acknowledges UGC, India for his fellowship. Support and help of Prof. Anunay Samanta and Mr. Navendu Mondal of School of Chemistry, University of Hyderabad in performing the transient spectroscopy experiments are gratefully acknowledged.


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Table of Contents Graphic

7.5

∆Α (mOD)

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390

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Wavelength (nm)


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Î±-Cyclodextrin Functionalized Carbon Dots: Pronounced

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