Facile synthesis of fluorescent graphene quantum dots from coffee grounds for bioimaging and sensing

Facile synthesis of ?uorescent graphene quantum dots from coffee grounds for bioimaging and

sensing

Liang Wang a ,Weitao Li a ,Bin Wu b ,Zhen Li c ,Shilong Wang b ,Yuan Liu a ,Dengyu Pan a ,?,Minghong Wu c ,?

a

School of Environmental and Chemical Engineering,Shanghai University,Shanghai 200444,PR China b

School of Life Science and Technology,Tongji University,Shanghai 200092,PR China c

Shanghai Applied Radiation Institute,Shanghai University,Shanghai 200444,PR China

h i g h l i g h t s

The highly ?uorescent PEI-GQDs are synthesized from coffee grounds. The PEI-GQDs exhibit enhanced band-edge photoluminescence property. The PEI-GQDs serve as low-cytotoxicity biological imaging agents. The PEI-GQDs show high selectivity for detecting Fe

3+

and Cu 2+.

a r t i c l e i n f o Article history:

Received 31January 2016

Received in revised form 15April 2016Accepted 22April 2016

Available online 24April 2016Keywords:

Graphene quantum dots Functionalization Bioimaging Sensing

a b s t r a c t

We have developed a novel eco-friendly approach involving hydrazine hydrate-assisted hydrothermal cutting followed by functionalization with poly (ethylene imine)(PEI)for fabricating highly ?uorescent graphene quantum dots from coffee grounds.The PEI-functionalized graphene quantum dots exhibit enhanced band-edge photoluminescence with single exponential decay.Their preliminary applications in bioimaging and sensing of heavy metals have been demonstrated.

ó2016Elsevier B.V.All rights reserved.

1.Introduction

Fluorescent nanomaterials have generated many applications in sensing [1,2],imaging [3–5],and optoelectronic devices [6,7]because of their unique optical properties.In past 20years,efforts have been focused on the synthesis of semiconductor quantum dots (SQDs)with size-dependent emission wavelengths and high quantum yields (QYs)[8].However,many of them were synthe-sized from toxic compounds such as CdX (X =S,Se,Te)and thus their large-scale applications would raise some potential environ-mental concerns [9].Therefore,it is still in high demand to synthe-size non-metal quantum dot materials for the above-mentioned https://www.sodocs.net/doc/2115088921.html,pared with SQDs,graphene quantum dots (GQDs)possess their favorable attributes without incurring the burden of intrinsic toxicity [2,4,10–16].As a consequence,a tremendous amount of researches has been devoted to the devel-opment of their applications in bioimaging [3,4,17–22],biosensing [23–25],light emitting diodes [26,27],solar cells [7,28],and photo-catalysis [11,29].It is thus of critical importance to exactly control their size (both diameter and layer number)and surface and edge chemistry during fabrication processes,because all these structural parameters will determine their electronic and optical properties associated with their various applications.

Towards this target,a lot of synthesis methods for various func-tionalized and non-functionalized GQDs have been established in recent ?ve years [2–4,10–30].Some involved chemical or electro-chemical cutting using carbon precursors from nanocarbons (gra-phene sheets [31–33]and carbon nanotubes [34])to bulk graphite [35]or counterparts such as arti?cial graphite [36,37],carbon ?bers [38]and coal [39,40].Apart from the top–down methods,a bottom–up strategy has also been developed using var-ious aromatic molecules [41–44].Some toxic organic small mole-cules have been employed to fabricate GQDs using multistep

https://www.sodocs.net/doc/2115088921.html,/10.1016/j.cej.2016.04.123

1385-8947/ó2016Elsevier B.V.All rights reserved.

?Corresponding authors.

E-mail addresses:dypan617@https://www.sodocs.net/doc/2115088921.html, (D.Pan),mhwu@https://www.sodocs.net/doc/2115088921.html, (M.Wu).

oxidative condensation reactions in organic solutions.In most cases,however,these organic synthetic methods suffer from some disadvantages,such as harsh reaction conditions,high prices, tedious processes,and the use of toxic starting materials[41–44]. In this regard,searching for precursors from regular food may pro-vide green routes that could overcome the above mentioned draw-backs.On the other hand,some kinds of natural food have been consumed by human beings for centuries,and now are still very common in daily life.If natural food could be re-used as a non-toxic starting material for the synthesis of GQDs,such eco-friendly synthesis would be valuable.Recently there are few reports on fabricating?uorescent QDs from natural food such as rice husk[22],honey[45],glucose[46],milk[47],orange juice [48],due to their low cost,easy availability,and nearly unlimited resource.These results inspired us to further produce?uorescent GQDs from natural food.Despite instant coffee or coffee bean have been used to prepared?uorescent QD materials for cell-imaging [49,50],then there are low PL QYs limited their applications in many?elds.Herein,we present a food route to?uorescent GQDs by cutting coffee grounds as the starting material without strong acid or expansive catalyst.The GQDs were further functionalized by poly(ethylene imine)(PEI).The functionalization is not only able to greatly enhance the?uorescence yield of GQDs,but also decreases the cytotoxicity of the GQDs at higher concentrations. The applications of PEI-functionalized GQDs(PEI-GQDs)for bioimaging and sensing are explored.

2.Experimental section

2.1.Preparation of GQDs and PEI-GQDs

Coffee beans were purchased from ZhongkaòCoffee Food Co. Ltd.Hydrazine hydrate and microporous membrane(0.22mm) were bought from Shanghai Chemical Reagent Co.Ltd.Poly(ethy-lene imine)was supplied by Alfar Aesar(1800,30%w/v).The dial-ysis bag with retained molecular weight of3500Da was bought from Beijing Dingguo Changsheng Biotechnology Co.Ltd.Deion-ized water were produced using a Milli-Q system (R>18.1M X cm).

GQDs were prepared from used coffee grounds of coffee beans through a hydrothermal route.Firstly,coffee beans were crushed by grinder and changed to hot coffee through Philipsòespresso https://www.sodocs.net/doc/2115088921.html,ed coffee grounds were cleaned and dried in an oven at80°C.In a typical procedure for synthesizing GQDs,0.1g coffee grounds and1ml hydrazine hydrate were dissolved into10ml water in an ultrasonic bath for30min.Then the solution was

transferred into a25ml Te?on lined stainless autoclave.The sealed autoclave was heated to150–200°C in an electric oven and kept for additional6–10h.After cooled to room temperature,the pro-duct containing water-soluble GQDs was?ltered through a

0.22mm microporous membrane to remove insoluble carbon pro-

duct,and further dialyzed in a dialysis bag for2days to remove unfused small molecules.The puri?ed black GQDs were dried at 80°C with a yield of33%,and further used for structural character-ization and property measurement.The synthetic procedure of PEI-GQDs was similar to that of the GQDs except for the addition of PEI in the media:A10ml GQD solution with the GQD concentrations of800mg Là1was mixed with0.1g of branched PEI solution under magnetic stirring,and then transferred to a25ml Te?on lined stainless autoclave.After heated at120°C for10h,the suspension was cooled naturally to room temperature.

2.2.Structural characterization

Samples were characterized by AFM using a SPM-9600atomic force microscope,TEM on a JEOL JEM-2010F electron microscope operating at200kV,X-ray powder diffraction(XRD)with a Rigaku D/max-2500using Cu Ka radiation,FT-IR spectroscopy recorded on

a Bio-Rad FTIR spectrometer FTS165,and Raman spectroscopy on a

Renishaw in plus laser Raman spectrometer with633nm.Absorp-tion and?uorescence spectra were recorded at room temperature on a Hitachi3100spectrophotometer and a Hitachi7000?uores-cence spectrophotometer,respectively.X-ray photoelectron spec-troscopy(XPS)data were obtained with an AMICUS electron spectrometer from SHIMADZU using300W Al K a radiation,and the binding energy calibrations were based on C1s at286eV,N 1s at400eV and O1s at532eV.The base pressure was about 3?10à9mbar.The PL QY of GQD and PEI-GQDs aqueous solutions were determined by comparing the integrated PL intensities (excited at370and390nm respectively)and the absorbency val-ues using9,10-bis(phenylethynyl)anthracene in cyclohexane as the reference.

2.3.Cell imaging

Hela cells were cultured in Dulbecco’s modi?ed Eagle(DMEM) medium(Gibco,USA)with L-glutamine,penicillin/streptomycin (Gibco,USA),10%(v/v)fetal bovine serum(Gibco,USA).Approxi-mately2?105Hela cells were seeded in culture dishes(diameter: 40mm)and cultured using the same culture medium(2ml per dish)at37°C under5%CO2/95%air.All cells were incubated for at least24h until$80%con?uence was reached.A GQD aqueous solution was introduced to the cells with a?nal concentration of H

y

d

r

o

t

h

e

r

m

a

l

t

r

e

a

t

m

e

n

t

GQDs powder

GQD colloid

P

E

I

,

1

2

o

C

Coffee beans

Coffee

Coffee grounds

Hydrothermal

NH

2

Scheme1.procedures of GQDs

76L.Wang et al./Chemical Engineering Journal300(2016)75–82

$40mg Là1in the culture medium.The cells were examined under

a confocal microscope(Leica TCS SP5)using lasers of405nm.

2.4.Cytotoxicity assay

Cell viability assays were performed using a MTT assay with3-(4,5-dimethyl-thiazole-2-yl)-2,5-phenyltetrazolium bromide.The cell viability was expressed as the percentage of viable cells in total cells.Hela cells were seeded in96-well plates(5?103cells per well)and pre-cultured in medium containing the PEI-GQDs at a dose of40,80and160mg Là1for24and48h.The cell viability of each well was treated with20l l of MTT solution(5mg mlà1)and incubated at37°C for4h.150l l DMSO was added after the medium was removed and the absorbance of each well at 490nm was measured using a Microplate Reader(Thermo,USA).

2.5.Determination of Fe3+and Cu2+

The following metal ions were chosen to evaluate the in?uence of metal ions on?uorescence of GQDs and PEI-GQDs,and assess the selectivity of Fe3+and Cu2+based on?uorescence variation: Al3+,Fe3+,Cr3+,Cu2+,Ni2+,Co2+,Mg2+,Zn2+,Cd2+,Mn2+,Pb2+,Ba2+, Ca2+,K+,Na+,and Li+.For selectivity of the GQDs and PEI-GQDs toward metal ions,the concentrations of the chosen metal ions

images and thickness distributions of GQDs(a)and PEI-GQDs(b).(c and d)TEM and diameter distributions of GQDs(c) and PEI-GQDs(f).Insets in e and f panels:fast Fourier transform(FFT)patterns of GQDs and PEI-GQDs.

L.Wang et al./Chemical Engineering Journal300(2016)75–8277

were10mM.For the quantitative measurement of Fe3+and Cu2+,a series of FeCl3,CuSO4,GQDs and PEI-GQDs solutions were pre-pared,in which the concentration of PEI-GQDs were kept identical and the concentrations of Fe3+and Cu2+varied from0to1l M. Their?uorescence intensities were determined using a?uores-cence spectrometer after5min of mixing.

3.Results and discussion

Scheme1outlines the main preparation and functionalization processes of the?uorescent GQDs from coffee grounds.Typically, coffee grounds were isolated from ground coffee beans,washed with deionized water,and dried.The coffee grounds were then treated at200°C under hydrothermal conditions.The resultant GQDs were further functionalized by PEI under hydrothermal con-ditions at a lower temperature(120°C)to avoid the decomposition of the PEI molecules.A light-brown supernatant of PEI-GQDs was isolated from the product by?ltration through a0.22l m microp-orous membrane.

The thickness analysis of the GQDs and PEI-GQDs was per-formed using AFM in Fig.1a and b.Their average heights are 2.23±0.54and3.18±0.74nm,respectively,suggesting that most of the GQDs and PEI-GQDs consist of several graphene layers.Both of the as-prepared GQDs are uniform and well dispersed in narrow size distributions(Fig.1c and d)with mean diameters of 1.88±0.72and2.67±0.81nm.The average size of the PEI-GQDs is larger than that of the GQDs due to PEI-functionalization on the surface.Their typical high-resolution TEM(HRTEM)images are shown in Fig.1e and f.The HRTEM contrast indicates that upon the PEI functionalization,several GQDs are self-assembled into lar-ger GQDs.The observation of the6-fold symmetrical FFT patterns indicates that these GQDs have the nature of graphene.A lattice spacing of0.22nm is ascribed to that of graphene(100)facets [30].These results suggest that the PEI-GQDs are composed of gra-phitic sp2carbon clusters.

Fig.2a shows the XRD patterns of the GQDs and PEI-GQDs.The nearly identical(002)interlayer spacing of the GQDs and PEI-GQDs is4.57and4.41?,https://www.sodocs.net/doc/2115088921.html,pared with the GQD (002)diffraction peak,that of the PEI-GQDs becomes more nar-rowed because of the fusion of smaller GQDs into larger ones dur-ing the PEI functionalization.Raman spectra(Fig.2b)further con?rm the quality of the as-prepared GQDs and PEI-GQDs.Both GQDs show the disordered(D)band at1351cmà1,related to the presence of sp3defects,and the crystalline(G)band at 1571cmà1,related to the in-plane vibration of sp2carbon.The ratio of the intensities(I G/I D)of these characteristic bands can be used to correlate the structural properties of the graphene.The I G/I D is1.09and1.25for GQDs and PEI-GQDs,respectively.These results indicate that PEI-GQDs may have fewer defects than GQDs. FT-IR spectra(Fig.2c)were used to identify the surface functional groups present on GQDs and PEI-GQDs.The broad absorption bands at3300–3500cmà1are assigned to stretching vibrations of O A H and N A H[51].The peak at2918cmà1in spectra shows the stretching vibrations of C A H[51].Those indicate that there are lots of amino and hydroxyl groups on the surface of the two GQDs, which results in these GQDs having a good hydrophilic nature. The bands at1650,1370and1076cmà1are attributed to the vibra-tional absorption band of C@O,C A O(carboxy)and C A O(alkoxy), respectively,[52]and the bands at1435cmà1are from the bending vibrations of N A H[53].X-ray photoelectron spectroscopy(XPS) was performed the further analysis of the GQDs and PEI-GQDs. The full scan XPS spectrum of GQDs and PEI-GQDs,as shown in Fig.2d,all of them presents three peaks at286,400,and532eV, which corresponds to C1s,N1s and O1s,respectively.This indi-cates that all of the GQDs and PEI-GQDs are doped with N from hydrazine hydrate and PEI.The high-resolution C1s spectrum (Fig.2e)reveals the strong signal of C A C at284.1eV,C A O at 286.4eV and the distinguishable C@O peak at288.1eV.Only one peak at399.6eV in the N1s XPS spectrum is assigned to N A H bonding of the GQDs and PEI-GQDs(Fig.2f).Thus,the primary PEI molecules play dual roles in the hydrothermal process:as the precursor for N-dopant and as the passivation agent,which both greatly contribute to the PL enhancement of PEI-GQDs,as observed

Table1

Atomic ratios determined from the full-range XPS spectra of GQDs and PEI-GQDs.

Atomic ratios(%)C1s N1s O1s C1s/O1s N1s/O1s

GQDs51.65 4.2444.11 1.170.096

PEI-GQDs53.74 5.4540.81 1.320.13

78L.Wang et al./Chemical Engineering Journal300(2016)75–82

in Scheme1under UV excitation.The peaks at532.8eV in the O1s spectrum(Fig.2g)are assigned to the O A H https://www.sodocs.net/doc/2115088921.html,pared with the atomic ratios of GQDs shown in Table1,the C1s/O1s and N1s/O1s of PEI-GQDs were increased the0.15and0.034, respectively,indicating that hydroxyl groups on the surface of GQDs were replaced by PEI through a low-temperature hydrother-mal route.

The optical spectra of colloidal aqueous solutions of the GQDs and PEI-GQDs are shown in https://www.sodocs.net/doc/2115088921.html,pared with the GQDs, the PEI-GQDs exhibit pronounced band edge absorption bands near390nm.Moreover,the PEI-GQD sample emits cyan?ores-cence at480nm with a high QY of24%on optical excitation at 390nm,while the GQD sample emits blue?uorescence at 460nm with a low QY of8%on optical excitation at370nm(the QYs were measured using9,10-bis(phenylethynyl)anthracene in cyclohexane as a reference).The corresponding full width at half maximum of the two PL spectra is as narrow as72and65nm, respectively,substantially smaller than that of previously reported GQDs[54](usually>100nm).The excitation wavelength depen-dence of the emission wavelength and intensity is a common phe-nomenon observed in carbon-based?uorescent materials[55–58]. These behaviors may re?ect not only effects from particles of dif-ferent sizes in the sample,but also distribution of different emis-sive sites on each nanoparticle.When excited by a series of monochromatic lights within the strong excitation band(310–430nm),there are some signi?cant differences in optical proper-ties between GQDs and PEI-GQDs.The emission wavelength of GQDs is strongly excitation-dependent(Fig.3b).The PL maximum of the GQDs shows large red shift of70nm(Fig.3b).By similar excitations of the GQDs,the emission wavelength of PEI-GQDs is nearly excitation-independent(Fig.3c).The PL maximum has a distinct small red shift of only10nm(Fig.3c).The less excitation-independent emission of the PEI-GQDs implies that both the size and the surface state of those sp2clusters contained in GQDs should be more con?ned.The PL intensities of the GQDs and PEI-GQDs are increased with enhancing the excitation wave-length?rstly(Fig.S1in Supplementary section),and achieve the largest PL intensity at the excitation wavelength of370nm and 390nm,respectively,then decline with increasing the excitation wavelength.The optimum excitation wavelengths of the GQDs and PEI-GQDs are370nm and390nm,respectively,which is con-sistent with the results shown in Fig.3a.Fig.3d and e shows their PL decay curves with a monoexponential decay feature.The decay were recorded for the GQD and PEI-GQD transitions at460nm for

L.Wang et al./Chemical Engineering Journal300(2016)75–8279

GQDs and 480nm for PEI-GQDs emission excited at 370nm and 390nm,respectively.The observed lifetimes of the GQDs was s =1.81ns,whereas for PEI-GQDs lifetime s =4.06ns was observed.The observed lifetime of GQDs and PEI-GQDs in nanosec-ond suggests that the synthesized GQDs and PEI-GQDs are most suitable for biological and optoelectronic applications.The PEI-GQDs also exhibit more stable PL than GQDs against the variation of pH of the solutions from strongly base to strongly acidic (Fig.3f).We believe that the PEI stabilizer plays a key role in the PL stability against pH values.Because the amine groups in the PEI are always charged from pH =4to 11,PEI-GQDs are also always charged regardless of pH values.The pH dependence effects of PEI-GQDs were also proved with the zeta potential as shown in Fig.S2in Sup-plementary section ,the zeta potential values of the PEI-GQDs were slightly decreased with the pH values ranging from 4to 11due to adsorbing some OH àion on the surface of PEI-GQDs.

Like ?uorescent carbon nanoparticles,?uorescent GQDs are also attractive for biomedical imaging because of their stable PL,low cytotoxicity,and excellent biocompatibility [3,4,17–19].Herein,we tentatively explore the use of the PEI-GQDs and GQDs as a ?u-orescent probe for imaging Hela cells.Hela cells were cultured in an appropriate culture medium containing the PEI-GQDs and GQDs at a low dose of 40mg L à1,respectively,and confocal microscopic images were taken with an excitation wavelength of 405nm and

10 μm

10 μm

10 μm

b

a c

24 h 48 h

[PEI-GQDs] (mg L -1)

d 8040100

C e l l V i a b i l i t y (%)

4080160

6020

PEI-GQDs using Hela cells.(a and c)Confocal ?uorescence image at 405nm excitation image (a),bright ?eld image (b)internalized into the cytoplasm of the cells.(d)Cytotoxicity assessment of PEI-GQDs at higher doses for incubation time PEI-GQDs

GQDs b

P L I n t e n s i t y (a . u .)

400

5000.0

1 μM

Fe 3+

C Fe / μM

(F 0-F )/F 0

0.40.8

0.0

0.40.8R 2= 0.996Wavelength (nm)

c

L I n t e n s i t y (a . u .)

0.0

01 μM

Cu 2+(F 0-F )/F 0

0.40.60.0

0.2relative PL intensities of PEI-GQDs between the blank and and Cu 2+(c).

80L.Wang et al./Chemical Engineering Journal 300(2016)75–82

the detection wavelength in the440–520nm range.The cytoplasm of cells display enhanced?uorescence around their nucleus after injecting the probe solution(Fig.4a and c and Fig.S3in Supple-mentary section),which indicates that both GQDs and PEI-GQDs penetrate into the cells and remain emissive.The brighter imaging for the PEI-GQDs is ascribed to their higher QY.

The cytotoxicity of the PEI-GQDs is also evaluated via MTT assays.Fig.4d shows the evolution of cell viability within24and 48h at doses of40,80and160mg Là1.At the imaging dose of 40mg Là1,the cell viability keeps over88%after24h incubation, revealing no cytotoxicity at low doses.However,for higher doses, the PEI-GQDs still show marked cytotoxicity.For example,the cell viability is reduced to62%at4-times imaging dose.This prelimi-nary result indicates that the?uorescent PEI-GQDs can serve as a low-cytotoxicity biological imaging agent.

Apart from the bioimaging functionality based on their PL prop-erty,the sensing functionality of the GQDs and PEI-GQDs was also evaluated as a?uorescence probe for ion detection for environ-ment analysis.Screening experiments with16metal ions (10mM)were carried out by investigating the PL intensity changes of the GQDs and PEI-GQDs,as shown in Fig.5a.Among the detected16ions,only Fe3+and Cu2+ions can quench the PL of the GQDs and PEI-GQDs.The PL quenching mechanism may be associated with the strong binding af?nity and fast chelating kinet-ics of Fe3+and Cu2+ions with N functional groups of GQDs and PEI-GQDs[59–61].The PL of the PEI-GQDs was lower than that of GQDs after adding the Fe3+and Cu2+ions due to much more N functional groups of PEI-GQDs than that of GQDs.By the chelating interaction, the irradiation combination of photoexcited electron-hole couples in the PEI-GQDs is inhibited through a fast electron transfer pro-cess from the PEI-GQDs to Fe3+or Cu2+ions.To give a quantitative analysis of Fe3+and Cu2+ions,the?uorescence spectra of the PEI-GQD solution containing different concentrations of Fe3+and Cu2+ ions were measured at the excitation wavelength of390nm (Fig.5b and c),and the quenching ef?ciency,(F0àF)/F0,was thus determined,where F0and F represent the PL intensity at blank and concentration of Fe3+or Cu2+,respectively.There is a good lin-ear relationship between the quenching ef?ciency and the detected ion concentration of Fe3+and Cu2+in the range from0 to1l M(see the inset of Fig.5b and c).These results show that the PEI-GQDs are very promising for Fe3+and Cu2+detection in practical applications.

4.Conclusions

In summary,we have developed an eco-friendly and simple route to prepare?uorescent GQDs functionalized by PEI using cof-fee grounds as the precursor.It is highly desirable to recycle food wastes for fabrication of multifunctional?uorescent materials via the approach.The PEI-GQDs show a highly ef?cient?uorescent property and a low cytotoxicity in favor of bioimaging applications. Moreover,the?uorescent probe is also highly sensitive to Fe3+and Cu2+https://www.sodocs.net/doc/2115088921.html,ing this marked ion-quenched PL characteristic,Fe3+ and Cu2+ions can be detected at a high sensitivity.Other applica-tions of the PEI-GQDs will be explored in the future such as opto-electronic devices and photocatalysis.

Acknowledgments

This work has been sponsored by National Natural Science Foundation of China(No.11174194,91233102),the Shanghai Sail-ing Program(No.16YF1404400),the Science and Technology Com-mission of Shanghai Municipality(No.16ZR1412100),the Key Laboratory for Advanced Displays and System Application,Min-istry of Education(Shanghai University),the Program for Chang-jiang Scholars and Innovative Research Team in University(No. IRT13078).

Appendix A.Supplementary data

Supplementary data associated with this article can be found,in the online version,at https://www.sodocs.net/doc/2115088921.html,/10.1016/j.cej.2016.04.123. References

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