Materials Science and Engineering C 58 (2016) 103–109

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Materials Science and Engineering C

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Phenylboronic acid functionalized reduced graphene oxide basedfluorescence nano sensor for glucose sensing

SK Basiruddin, Sarat K. Swain ⁎Department of Chemistry, Veer Surendra Sai University of Technology, Burla, Sambalpur 768018, India

⁎ Corresponding author.E-mail address: swainsk2@yahoo.co.in (S.K. Swain).

http://dx.doi.org/10.1016/j.msec.2015.07.0680928-4931/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 May 2015Received in revised form 4 July 2015Accepted 31 July 2015Available online 6 August 2015

Keywords:Photo-luminescenceGrapheneFluorescentNano-sensor

Reduced graphene has emerged as promising tools for detection based application of biomolecules as it has highsurface area with strong fluorescence quenching property. We have used the concept of fluorescent quenchingproperty of reduced graphene oxide to the fluorescent probes which are close vicinity of its surface. In presentwork, we have synthesized fluorescent based nano-sensor consist of phenylboronic acid functionalized reducedgraphene oxide (rGO–PBA) and di-ol modified fluorescent probe for detection of biologically important glucosemolecules. This fluorescent graphene based nano-probe has been characterized by high resolution transmissionelectronmicroscope (HRTEM), Atomic forcemicroscope (AFM), UV–visible, Photo-luminescence (PL) and Fouri-er transformed infrared (FT-IR) spectroscopy. Finally, using this PBA functionalized reduced GO based nano-sensor, we were able to detect glucose molecule in the range of 2 mg/mL to 75 mg/mL in aqueous solution ofpH 7.4.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Reduced graphene has emerged as promising tools for detectionbased application of biomolecules. Graphene oxide is a two-dimensionalsheet like nanostructured carbon based nanomaterials. It is composedof mainly sp3 and sp2-hybridized carbon atoms with various func-tional groups like –COOH, –OH and epoxide on their surface. WhereasGraphene, a single-atom-thick monolayer containing sp2 carbon atomspresent in conjugated form in a sheet like structure. Graphene has highsurface area with high electronic conductivity and strong fluorescencequenching property which is being used for electrical [1] and optical [2]detection of various biomolecules. D-Glucose is an elementary necessityof living creatures and a ubiquitous bio-fuel for many biological practices.Glucose level is maintained by its own biological process. Abnormality ofglucose level in blood leads to disease like diabetes. Therefore sensing anddetermination glucose level in blood has become emerging field for thediagnosis of diabetes. Many methods have been published on sensing ofglucose molecules like electrochemical sensing [3–6], Quantum dotsimmobilizedmicrogel based optical sensor [7], droplet-basedmicrofluidicelectrochemical sensor [8], photonic crystal based naked eye glucosesensing [9], liquid crystal based glucose sensing [10], aggregation inducedfluorescent enhancement and quenching based sensing [11,12], andmany other enzymatic [13] and non-enzymatic based glucose sensing[14–18]. Detection of glucose using phenylboronic acid functionalizedvarious scaffolds has been reported [3,12,19–22]. A few papers haveshown fluorescence based glucose sensing using graphene oxide based

nano probe [23–25]. Preparation of graphene oxide based enzymaticallyelectrochemical glucose sensor are costly as well as handling of enzymeis not easy. Electrochemical catalytic activity of graphene based for elec-trochemical sensing of glucose is poor. Therefore for better catalytic activ-ity graphene based materials are made composite with other metals ormetal alloys nanoparticles [4,6,15,17]. A few papers have been reportedfor glucose detection using fluorescent functionalized graphene quantumdots [11,23]. But in this case, further chemical modification of graphenebased materials is needed for fluorescent graphene quantum dots prepa-ration. Hao Zhang et al. reported turn on fluorescence based sensor forglucose determination using graphene oxide–DNA scaffold [24]. There-fore development of properly affinity molecules functionalized grapheneoxide based non enzymatic and fluorescence based glucose sensor needto be explored for the sake of scientific community especially those whoare working in the field of bio medical science. Here, in this paper wehave used phenylboronic acid functionalized reduced graphene oxide(rGO–PBA) for fluorescence based detection of glucose. We developedan easy and simple cost-effective synthetic method for preparation ofPBA functionalized reduced graphene oxide based nano probe.Finally, using this nano probe glucose has been detected in therange of 2 mg/mL to 75 mg/mL at pH 7.4. It's a fluorescent basedenzyme-free detection of glucose in solution.

In this work, we used the concept of fluorescent quenching prop-erty of reduced graphene oxide of a fluorescent probes which areclose vicinity of its surface. Phenylboronic acid is well known forhaving preferential affinity to diol containing molecules throughcyclic ester bond formation. Therefore we prepared a diol modifiedfluorescent probe for specific interaction with phenylboronic acid.That's why, we have used phenylboronic acid functionalized reduced

Scheme 1. Synthesis of di-ol modified florescent probe (c) starting from 5-(2-Amino-ethylamino)-naphthalene-1-sulphonuc acid (ANSA) (a) and 2,3-dihydroxy-propionaldehyde orglyceraldehyde (b) by reductive amination method.

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graphene oxide (rGO–PBA) and di-ol modified fluorescent probe(Scheme 1-c) for fluorescent based detection of glucose molecule.First, 3-aminophenylboronic acid was conjugated to graphene oxidesurface followed by the hydrazine reduction and a fluorescencemolecule(5-(2-Amino-ethylamino)-naphthalene-1-sulphonuc acid) was conju-gated to the glyceraldehyde by reductive aminationmethod to synthesizedi-ol modified fluorescent probe (Scheme 1-c). On addition of diol mod-ified fluorescent probe to the PBA functionalized reduced GO solution;phenylboronic acid on the reduced graphene oxide forms a cyclicboronate ester with fluorescent probe. Therefore, fluorescent probecome close to the reduced GO surface and hence the fluorescence isquenched (Scheme 2). Upon addition of glucose molecule to thequenching state of di-ol modified fluorescent probe in the cyclic esterform with PBA of the rGO–PBA, glucose forms new cyclic boronate esterwith rGO–PBA and replace the di-ol modified fluorescent probe fromthe reduced GO surface. As a result fluorescence property of the di-olmodified fluorescent probe comes back from its quenched state. In thisnon-enzymatic and cost effective approach, the PBA functionalized re-duced graphene oxide (rGO–PBA) fluorescence based sensor has beenused to detect glucose molecule in the limit of 2 mg/mL to 75 mg/mL.

2. Experimentals

2.1. Materials

Graphite powder, 3-aminophenylboronic acid (APBA) (MW136.96), glyceraldehyde, glucose, dextran (MW 6000), 5-(2-Amino-ethylamino)-naphthalene-1-sulphonuc acid (ANSA), hydrazinemonohydrate, NH3 solution, Alizarin red S. (ARS) all were purchasedfrom Sigma-Aldrich and used as received.

Scheme 2. Fluorescence based sensing of glucose molecule using PBA functionalized reducedstrong fluorescence signal from the solution in presence UV light.

2.2. Instruments

The UV–vis absorption spectra were recorded on a ShimadzuUV-2550 UV–vis spectrophotometer, and the fluorescence measure-ments were performed on a BioTek Synergy Mx microplate reader.High resolution transmission electron microscopy (HRTEM) imageswere obtained using a JEOL-JEM 2010 electron microscope. Atomicforce microscope (AFM) was measured using VEECO DICP II autoprobe(model AP 0100). Fourier transform infrared (FTIR) spectra of KBrpowder-pressed pellets were recorded on a Perkin-Elmer Spectrum100 FTIR spectrometer.

2.3. Synthesis of graphene oxide (GO)

Graphene oxide was synthesized by modified Hammer's method[26]. Typically, 200 mg graphite powder, 100 mg sodium nitrate and5 mL concentrated H2SO4 were mixed together in a 100 mL beakerand cooled to 0 °C. Then the solution was kept under vigorous stirring.Next, 600 mg KMnO4 was added to this solution in stepwise mannerso that the temperature was should not raise above 20 °C during theseKMnO4 addition steps. After the complete addition of KMnO4 the tem-perature of the solution was slowly raised to 35 °C and kept in this con-dition for 30min. A brownish gray pastewas formed. Next, 10mLwaterwas added to the whole solution and the solution turned brownish yel-low. The temperature of the solution was increased to 98 °C duringwater addition and this temperature was maintained for 15 min. Thewhole solution was then mixed with 28 mL of warm water followedby addition of 500mL 3%H2O2 that reduces the residual permanganate.The light yellow particles were washed thoroughly with warm water

graphene oxide (rGO–PBA). Here OFF stands for no fluorescence signal and ON stands for

Scheme 3. Synthesis of PBA functionalized reduced graphene oxide (rGO–PBA) by reacting epoxide group of GO with NH2 group of 3-aminophenylboronic acid.

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7–8 times. The solid was air dried and dissolved in 20mL distilled waterby 15 min sonication. Then it was centrifuged at 3000 rpm for 30 min.The supernatant was collected as graphene oxide (GO) solution andstored for further use.

2.4. Synthesis of dextran stabilized reduce graphene oxide

Graphene oxide (GO) (1 mg/mL) solution was prepared by themodified Hummer's method and coated with dextran according tothe reported method [26,27]. Typically, 400 mL of graphene oxidesolution was diluted with 2 mL of water. Then, 150 mg of soliddextran (6 kDa) was added and dissolved by stirring. Next, 50 μLof hydrazine monohydrate and 50 μL of ammonia solution (25%)were added and the temperature of the solution was maintainedat 80 °C with constant stirring. The reaction was continued for 1 h.Under these conditions graphene oxide is reduced to graphene (re-duced graphene oxide) and covalently linked with dextran. Then,solid NaCl was added to the reaction mixture until the complete pre-cipitation of graphene was observed. The precipitated dextran coat-ed reduced graphene oxide was washed thoroughly with water anddispersed in water via sonication for further use.

Fig. 1. FTIR spectra (A & B) of graphene oxide (GO), 3-aminophenylboronic acid (AP

2.5. Preparation of di-ol modified fluorescent probe

We synthesized di-ol modified fluorescent probe c by reacting glyc-eraldehyde with 6-(2-Amino-ethylamino)-naphthalene-1-sulphonucacid (ANSA) (b) by reductive amination method (Scheme 1) [28,29].Typically, 1 mL of 0.5 M aqueous solution of 6-(2-Amino-ethylamino)-naphthalene-1-sulphonuc acid was mixed with 1 mL of 0.5 M aqueoussolution of glyceraldehyde and stirred for 1 h. NH2 group of ANSA reactsaldehyde (–CHO) group of glyceraldehyde and forms imine bondduringreaction. Then aqueous solution of Na(CN)BH3 was added to reduceimine bond formed between ANSA and glyceraldehyde. In this way di-ol modified fluorescent probe has been synthesized.

2.6. Preparation of phenylboronic acid functionalized reduced grapheneoxide

Freshly prepared 8 mL of graphene oxide (0.5 mg/mL) solution wastaken in a 15 mL glass vial. Then 25 mg of 3-aminophenylboronic acidwas added to the graphene oxide solution and the reaction mixturewas vigorously stirred at temperature 70 °C for 3 h. Here under this con-dition phenylboronic acid functionalized graphene oxide is formed.

BA) and phenylboronic acid functionalized reduced graphene oxide (rGO–PBA).

Fig. 2. UV–visible spectra of alizarin red S (ARS) in presence of phenylboronic acid func-tionalized reduced graphene oxide (rGO–PBA) and dextran functionalized reduced GO(rGO-dextran).

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Next, 50 μL of hydrazine monohydrate and 50 μL of ammonia solution(25%) were added and the temperature of the solution was maintainedat 80 °C with constant stirring. The reaction was continued for 1 h. Inthis condition phenylboronic acid functionalized graphene oxide is re-duced to phenylboronic acid functionalized graphene oxide (rGO–PBA). Then, solid NaCl was added to the reactionmixture until the com-plete precipitation of reduced graphene was observed. The precipitateof phenylboronic acid functionalized graphene was washed thoroughlywith water to remove unreacted phenylboronic acid. This precipitationredispersion method was repeated for more two times and finally dis-persed in water via sonication for further use.

2.7. Fluorescence based sensing of glucose using phenylboronic acidfunctionalized reduced graphene oxide (rGO–PBA)

2 mL of phenylboronic acid functionalized reduced graphene oxide(0.1 mg/mL) was taken in PL (photoluminescence) cuvette. Then di-olmodified fluoresce probe was added to the PL cuvette containingphenylboronic acid functionalized reduced graphene oxide (rGO–PBA). In this study di-ol modified fluorescent probe has been excitedby the wavelength of 360 nm (λex ~360 nm) and emission spectra

Fig. 3. Transmission electron microscopic (TEM) images of as synthesized graphene oxide (G

were collected by photoluminescence spectrometer. Amount of addi-tion of di-ol modified fluoresce probe was such that the fluorescenceof the probe is just quenched by the reduced graphene oxide byboronate ester formation between the di-ol of fluorescent probe andphenylboronic acid of phenylboronic acid functionalized reducedgraphene and this fluorescent property of the solution was observedby exciting the solution by a UV lamp. Here fluorescent probe come tothe close vicinity of reduced graphene oxide by boronate ester forma-tion and fluorescence of the probe is quenched by the super quencherreduced graphene oxide. After that calculated amount of glucose (0 to75 mg/mL) was added to the probe solution (II). Sensitivity of this bio-sensor was in the range of 2 mg/L to 75 mg/L. Fluorescence is graduallycoming back from the quenched state due to the new boronate esterformation between glucose and phenylboronic acid of phenylboronicacid functionalized reduced graphene and di-ol modified fluorescentprobe gets far away from the graphene oxide surface leading the fluo-rescence property coming back.

3. Results and discussion

3.1. Materials characterization

Graphene oxide (GO) has been synthesized using modifiedHammer'smethod. Successfully synthesis of graphene oxide is reflectedby the transmission electronmicroscope (TEM) and atomic forcemicro-scopic (AFM) studies of graphene oxide (Figs. 3 & 4). As synthesizedgraphene oxide contains many functional groups like –COOH, –OH,and epoxide group. It is well known that epoxide group reacts withprimary amine groups to form –C–NH– bond and a hydroxyl group –C(OH)-. Here we prepared phenylboronic acid (PBA) functionalizedreduced graphene by reacting 3-aminophenylboronic acid withgraphene oxide by using epoxide-amine reaction pathway [30].NH2 groups of 3-aminophenylboronic acid (APBA) react with epox-ide groups of graphene oxide and form a –C–NH bond betweenAPBA and GO (Scheme 3).

The UV visible, FTIR, HRTEM and AFM are carried out to characterizegraphene oxide (GO) and PBA functionalized reduced graphene oxide.In FTIR spectra (Fig. 1) new peaks at 530 cm−1 and 700 cm−1 presentin PBA-rGO, are due to in-plane ring deformation vibrations and out-of-plane ring deformation vibrations (3 neighboring H atoms) of 1,3 di-substituted benzene ring of 3-aminophenylboronic acid, respectively.Peak at 1260 cm−1 is due to C–O stretching vibration of epoxide, thispeak intensity decreased in rGO–PBA in comparison with GO. Newpeak at 1435 cm−1 present in PBA-GO, is due to C–C stretch (phenyl

O) (A) and phenylboronic acid functionalized reduced graphene oxide (rGO–PBA) (B).

Fig. 4. AFM images of as synthesized graphene oxide (GO) (A) and phenylboronic acid functionalized reduced graphene oxide (rGO–PBA) (B) and their corresponding height profilediagrams A1 and B1, respectively.

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ring) and B–O stretching of APBA but this peak is absent in GO. The peakat 1620 cm−1 is due to C_C stretching of graphene oxide as well as re-duced graphene oxide. The relative intensity of this peak in r GO-PBA in-creases with respect to the C_O stretching observed at 1720 cm−1 ofacid group due to increase of population of the double bonds in reducedGO-PBA.

Alizarin red S (ARS) is a commercially available dye which shows adramatic change in optical property after interaction with phenylboronicacid. Herewehave usedARS to characterizewhether PBA is conjugated tothe graphene oxide surface or not. In UV–visible spectrum, Alizarin red S(ARS) gives two absorbance peaks at 335 nm and 425 nm in solution. Butwhen Alizarin red S form boronate ester it gives fluorescence at 590 nm(λex-450 nm) and absorbance at 500 nm [31]. In UV–Visible spectrum(Fig. 2) Alizarin red S gives two absorbance peaks at 335 nm and500 nm in presence of phenylboronic acid functionalized reduced

Scheme 4. Fluorescence based sensing of glucose molecule using PBA

graphene oxide (rGO–PBA) due to formation of boronate ester withphenylboronic acid group of rGO–PBA. On the other hand as a control ex-periment, Alizarin red S in dextran functionalized reduced grapheneoxide solution does not give any peaks centered at 500 nm but givesonly two peaks at 335 nm and 425 nm just like only ARS. Fluorescenceproperty of ARS in presence of rGO–PBA could not be detected dueto the presence of reduced GO. Actually here fluorescence ofboronate ester of ARS gets quenched by the close vicinity of reducedgraphene oxide to the boronate ester of ARS. TEM image (Fig. 3)showed no appreciable change of graphene oxide after functionalizationwith phenylboronic acid. We carried out the atomic force microscope(AFM) measurement to study the surface morphology of grapheneoxide (GO) and GO after functionalization with phenylboronic acid.AFM image (Fig. 4) showed that the width of PBA functionalized reducedGO slightly high that that of as synthesis graphene oxide. Conjugation of

functionalized reduced graphene oxide (rGO–PBA) schematically.

Fig. 5.Glucose detection using PBA functionalized reduced graphene oxide (rGO–PBA) nanoprobes (A). Figure B shows ((F–F0)/F0) versus concentration of glucose plot demonstrating thefluorescence intensity change with increase of glucose concentration (2 mg/mL to 75 mg/mL). F0 and F are fluorescence intensity of the PBA functionalized graphene based nano sensorbefore and after addition of glucose solution, respectively.

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PBA to theGO increases the height of the GO as PBA is covalently attachedto the graphene oxide surface. But, at the same time reduction ofgraphene oxide by hydrazine should decrease the height of GO surfaceas removing the epoxy and hydroxyl groups from both sides of GO sheetsresults in decreasing in thickness reduced graphene oxide. These two op-posite factors slightly increases the height of GO of rGO–PBA in AFMimage. This result quietly matches with the information of the publishedresults [32,33]. From the above experimental results it is clear thatphenylboronic acid has been successfully conjugated to graphene oxidesurface.

3.2. Fluorescence based sensing of glucose using r-GO-PBA based nanosensor

Detection of glucose was carried out using phenylboronic function-alized reduced GO and di-ol modified fluorescent probe (Scheme 4).First, di-ol modified fluorescent probe c which has two vicinal –OHgroups. On the other hand phenylboronic acid functionalized reducedgraphene oxide is prepared by well-known epoxide-amine reaction(Scheme 3). Phenylboronic acid has strong affinity to form a boronateester with the molecules which have vicinal OH groups. Phenylboronicacid functionalized reduced GO forms a boronate ester with di-ol mod-ified fluorescent probe and fluorescent molecule came close vicinity ofthe reduced GO and fluoresce gets quenched (or off). Now addition ofglucose molecule to the above system, glucose molecule forms a strongboronate ester with PBA functionalized reduced graphene oxide thanthat of the di-ol modified fluorescent probe. Hereby di-ol modified

Fig. 6. Control experiment dextran functionalized reduced graphene oxide (r-GO-dextran) as gPBA functionalized reduced graphene oxide (rGO–PBA) based nanoprobes (B).

fluorescent probes get free and go far away from the surface of the re-duced GO and fluorescence comes back or fluorescence is on. This quan-titative fluorescence signal is depends on the amount of glucose addedto the quenched state of the fluorescence. With increase of amount ofglucose fluorescence intensity gradually increases (Fig. 5). We did alsothe control experiment to detect glucose using dextran functionalizedreduced graphene oxide solution to confirm the uniqueness ofphenylboronic acid functionalized reduced GO for the detection of glu-cose in comparison with only reduced GO without functionalization ofPBA (Fig. 6A). Briefly, di-ol modified fluorescent was added to the dex-tran functionalized reduced GO solution. The fluorescence property ofprobe was quenched by the dextran functionalized reduced GO due tonon-specific interaction between reduced graphene oxide and di-olmodified fluorescent probe. On addition of glucose to the fluorescentquenched solution, no fluorescence was came back (Fig. 6A). In presentinvestigation, graphene oxide is covalently functionalized with PBA andsubsequently reduced by hydrazine as a result of which, it is subse-quently used as glucose sensing nanoprobes. Hence, the chemicalstate of graphene oxide was not affected by the glucose. The grapheneoxide was reduced by hydrazine. Further, the sensitivity of the sensorwas not hampered by the glucose.

Hence phenylboronic acid functionalized reduced GO is only effi-cient for fluorescent based detection of glucose but not the dextranfunctionalized reduced GO. Using the PBA functionalized reduced GOfluorescence based glucose sensor, we were able to detect minimum2mg/mL tomaximum75mg/mLglucose inwater of pH7.4.Weextend-ed our studies for detection of glucose based carbohydrate or other

lucose detection nanoprobes (A). Sensing of other glucose containing carbohydrate using

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carbohydrate using PBA functionalized reduced GO based nano sensor(Fig. 6B). We observed that PBA functionalized fluorescent nano sensorcan be also used for the detection of any glucose based carbohydratemolecule. But non glucose based carbohydrate molecules (e.g., galac-tose) gives veryweakfluorescence signal on addition to nano sensor so-lution in comparison to the glucose or glucose based carbohydrate (e.g.,glucose,maltose, lactose and dextran) (Fig. 6B). Therefore PBA function-alized reducedGObasedfluorescence based nano sensor can beused forspecific detection of glucose or glucose based carbohydrate molecules.

4. Conclusion

We have synthesized fluorescent based nano-sensor consist ofphenylboronic acid functionalized reduced graphene oxide and di-olmodified fluorescent probe for detection of biologically important glu-cose molecule. PBA functionalized reduced GO based fluorescencenano sensor also can be used for specific detection other glucosebased carbohydrate molecules. PBA functionalized reduced GO basednano-sensor were used for detection of glucose in the range of2 mg/mL to 75 mg/mL. This fluorescent based nano-sensor was easilysynthesized and can be stored for a period of long time for the detectionbased application. PBA functionalized reduced graphene oxide can beused as drug carrier in the drug delivery based application for variouscancer/tumor cells having over expressed of various carbohydrate totheir cell surface.

Acknowledgment

Authors acknowledge the financial support provided by UniversityGrants Commission (CH/13-14/0011), Govt. of India through the Dr D.S. Kothari Post-Doctoral Fellowship to SK Basiruddin.

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