Spectral Migration of Fluorescence in Graphene Oxide Aqueous

Dec 2, 2013 - resolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, that the migration is associated w...
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Spectral Migration of Fluorescence in Graphene Oxide Aqueous Dispersions: Evidence for Excited-State Proton Transfer Bharathi Konkena and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Aqueous dispersions of graphene oxide (GO) exhibit strong pHdependent fluorescence in the visible that originates, in part, from the oxygenated functionalities present. Here we examine the spectral migration on nanosecond timescales of the pH dependent features in the fluorescence spectra. We show, from timeresolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, that the migration is associated with excited state proton transfer. Both ‘intramolecular’ and ‘intermolecular’ transfer involving the quasimolecular oxygenated aromatic fragments are observed. As a prerequisite to the timeresolved measurements, we have correlated the changes in the steady state fluorescence spectra with the sequence of dissociation events that occur in GO dispersions at different values of pH. SECTION: Spectroscopy, Photochemistry, and Excited States

G

present (Figure 1b). The pK values of 4.3 and 6.6 correspond to ionization of aromatic carboxylic groups, but the former have hydroxyl groups present in an ortho position that allows the carboxylate anion to be stabilized by H-bonding and hence these groups are more acidic, a behavior similar to that observed in the hydroxynapthoic acids. The pK of 6.6 corresponds to isolated carboxylic groups while the pK of 9.8 corresponds to the ionization of the phenolic OH; here, too, the anion is stabilized by a phenolate to ketone transformation. The intrinsic fluorescence of GO and reduced-GO have been extensively investigated because of their potential technological significance.7,12 The fluorescence behavior of GO is complex it is different for dispersions and films,5 and even for dispersions depends on the pH as well as the excitation wavelength.13 The reported emission maxima, too, have varied quite significantly with the fluorescence reported to peak in the red and blue regions of the visible spectra, depending on the mode of preparation and the pH of the medium.13,14 There is, however, broad consensus, based on steady state and fluorescence decay measurements, that there are (at least) two bands with maxima between 350 and 450 nm and 500 and 800 nm, respectively.3,4,13,15 The two bands, however, have been studied and reported separately, and it is not clear whether and how the two are related. The physical basis of the fluorescence in GO is unclear with a number of unresolved issues. The proposed mechanisms fall broadly in two classes. The fact that the fluorescence wavelength is tunable by reduction/oxidation has been interpreted to indicate that emission originates from the recombination of electron−hole

raphene sheets, one-atom-thick, two-dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes: high electronic and thermal conductivities and exceptional mechanical strength.1,2 Graphene oxide (GO), the oxidized form of graphene, was primarily considered only as an easily available precursor for chemical routes to reduced graphene oxidea material that has many properties similar to the mechanically prepared graphene sheets. More recently it has been realized that GO is an interesting material in its own right. One such attribute that has attracted widespread attention is the intrinsic and tunable fluorescence in the visible3 and near-infrared (NIR)4,5 of the as-prepared GO. In combination with the excellent aqueous dispersibility, it offers a distinct advantage over other carbon based fluorophores, especially for biological applications such as live-cell imaging.4,6−8 GO sheets, prepared by the oxidation of graphite, contain a sizable fraction of carbons that are sp3 hybridized and covalently bonded to oxygen in the form of epoxy, hydroxyl, carbonyl and carboxylic functional groups.9 The remaining carbons form isolated sp2 graphene-like networks that are embedded in the sp3 matrix (Figure 1a). It is this heterogeneity at the nanoscale that makes interpretation of the properties of GO both intriguing and difficult. GO sheets form stable dispersions in aqueous media over a wide range of pH values and as shown from zeta-potential measurements, the stability of the dispersions are due to electrostatic repulsion between the negative charge on the sheets, as consequence of the ionization of the functional groups present.10,11 From pH-titrations, in conjunction with infrared spectroscopy, the underlying chemistry of the aqueous dispersibility of GO had been established.11 GO exhibits pK values of 4.3, 6.6, and 9.8 corresponding to ionization of the different functionalities © 2013 American Chemical Society

Received: November 16, 2013 Accepted: December 2, 2013 Published: December 2, 2013 1

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Figure 1. (a) STM image of GO on HOPG. The regions encircled by the blue line are disordered oxidized regions while the green encircled regions are holes or gaps within the sheet. (b) pK distribution of GO (from ref 11). The pH values at which fluorescence was measured are indicated. (c) Absorption and steady-state fluorescence spectrum of GO dispersions on excitation at 320 nm at pH values 2.5 (black), 5.5 (blue), 7.5 (red) and 10.6 (green). The fluorescence spectra of GO dispersions recorded at different excitation wavelengths from 260 to 400 nm at (d) pH 5.5 (e) pH 7.5 and (f), pH 10.6.

(e−h) pairs, associated with localized disorder induced defect states within the spatially disordered π−π* gap, of the nanosized nonoxidized sp2 graphene-like domains.3,5,7,12 A recent observation of subpicosecond spectral migration of fluorescence of GO films to longer wavelengths has been interpreted as arising from the energy redistribution within these localized emitting states.16 In contrast, the fact that the fluorescence of GO dispersions are structured and pH dependent have been interpreted as evidence of a quasimolecular origin of the fluorescence.14 The fluorophores are the oxygenated functionalities associated with the sp3 carbons of GO. The close similarity of the wavelength and decay times of the pH-dependent fluorescence of GO and those of aromatic compounds, especially the carboxylic acids, has been highlighted in many reports.14,17,18 A recent study has assigned the lower wavelength features in the fluorescence to aromatic groups bound to single oxygen functionality and the higher wavelength feature to aromatic moieties bound to several functional groups.18 The pH dependence has been attributed to dissociation of these groups and the fact that the pK values in the excited state can be quite different from that in the ground state.14 Like its tunability, the pH dependence of the fluorescence of GO, too, has been explored for potential applications.19 Considering the heterogeneous nature of GO, the presence of sp2 domains in coexistence with oxygen functionalities bound to sp3 carbons, it is unlikely that a single mechanism would be able to explain the entire gamut of fluorescence phenomena observed in GO. Here we examine the spectral migration on nanosecond time-scales of the pH dependent features in the fluorescence spectra, in the visible, of aqueous dispersions of GO. We show, from time-resolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, that the

migration is associated with excited state proton transfer. Both ‘intramolecular’ and ‘intermolecular’ transfer involving the quasi-molecular oxygenated aromatic fragments are observed. As a prerequisite to the time-resolved measurements, we have correlated the changes in the steady state fluorescence spectra with the sequence of dissociation events that occur in GO dispersions at different values of pH. The functional groups involved in these dissociation events had been established, previously.11 Graphene oxide was prepared by the modified Hummers procedure (see Supporting Information).20,21 The material was characterized by X-ray photoelectron spectroscopy, 13C magic angle spinning NMR, and Raman spectroscopy (details are provided as part of the Supporting Information). The N2adsorption BET surface areas of the as-prepared GO was 170 m2/g. Atomic resolution STM images of GO sheets deposited on HOPG were recorded on a Multimode Atomic force microscope (Veeco MMAFMLN_AM-2113) using a tungsten metal tip in the constant-current (2 nA) mode. Steady state (Horiba Jobin Yvon Fluoromax 400) and fluorescence decay (Fluoromax-4-TCSPC) measurements were recorded on dilute dispersions (0.05 mg/mL). The time-resolved emission spectra (TRES), I(λ,t), or the instantaneous fluorescence spectra, were constructed from the wavelength dependent decays together with the steady-state fluorescence spectrum.22 The procedure involves fitting the decays, Γ(λ,t), with the following multiexponential relationship: Γ(λ , t ) =

∑ ai(λ) exp[−t /τi(λ)]

(1)

where τi(λ) are the wavelength dependent decay times and ai(λ) (with ∑ai(λ) = 1), the pre-exponential factors. We have used up to four decay times, i = 1−4, to analyze the decay curves, but because of parameter correlation, no molecular 2

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Scheme 1. Sequence of Ionization Events for GO in Aqueous Dispersions at Different Values of pHa

a

The emission wavelengths of the ionized species are indicated.

dispersions change with the pH of the medium (Figure 1c).14 There are, however, no changes in the corresponding absorption spectra of the dispersions with pH; the band at 224 nm and the shoulder at 300 nm may be assigned to π−π* transition of the sp2 network and the carbonyl n-π* band, respectively.10 The changes in the fluorescence spectra are discussed in greater detail by examining the spectra as a function of excitation wavelength (Figure 1d−f) at the different values of pH. The fluorescence spectra of GO dispersions at pH 2.5 show a broad band in the green at 550 nm that shows no change with excitation wavelength (see Supporting Information). This band is characteristic of carboxylic groups in their protonated state.25,26 The spectra at pH 5.5 show two bands with relative intensities that depend on the excitation wavelength. The blue emission band at 430 nm is most intense when excited at 320 nm. It also exhibits a small red shift from 410 to 430 nm on change in the excitation wavelength from 260 to 320 nm. The green emission band at 550 nm shows maximum intensity on excitation at higher wavelengths, 360 nm. We assign the 430 nm band to emission from ionized carboxylate groups that are ortho to a hydroxyl group. As identified previously, these are the groups that are dissociated at pH 5.5. The 550 nm band is assigned to carboxylic groups that remain undissociated at this pH. At pH 7.5, the behavior is similar to that at pH 5.5; a blue emission at 430 nm that appears at low wavelength excitation and a green emission at higher excitation wavelengths. The main difference as compared to the spectra at pH 5.5 is that the green emission is blue-shifted to 515 nm at the higher pH. This is because of the dissociation of the carboxylic groups that were responsible for the 550 nm emission at lower values of pH (>6.6). It is well-known that protonation causes a red shift in the fluorescence of aromatic carboxylic acids, and a similar explanation is probably true in GO dispersions.27 The

significance can be assigned to these parameters. The TRES, I(λ,t), may then be constructed from the steady state fluorescence spectra, Iss(λ), I(λ , t ) = Iss(λ) ∑ ai(λ) exp[−t /τi(λ)]/ τ(λ)

(2)

where ⟨τ(λ)⟩, the average decay time, is given by ⟨τ(λ)⟩ = Σai(λ)·τi(λ). The time-resolved area normalized emission spectra (TRANES) are a modified version of the TRES, and are obtained by normalizing the area of each spectrum in the TRES such that the area of the spectrum at time, t, is equal to that of the spectrum at shorter times.23,24 The TRANES, in addition to providing a different representation of the timeresolved data, is useful in diagonizing whether multiple excited species, if present, are in equilibrium.24 Atomic resolution STM images of the as-prepared GO sheets (Figure 1a) clearly highlights the disordered nature of the material. The presence of unoxidized sp2 patches, oxidized regions, and holes may clearly be seen. Here our focus is on the fluorescence associated with the oxidized sp3 regions. These functionalties are also responsible for the aqueous dispersibility of GO. The steady state fluorescence spectra were recorded for GO dispersions at pH values 2.5, 5.0, 7.5, and 10.6. As reported earlier, the pK values of GO dispersions show a distribution (Figure 1b; reproduced from ref 11) with three main features at pK values of 4.3, 6.6, and 9.8. The pK values of 4.3 and 6.6 correspond to dissociation of aromatic carboxylic groups, but the former have hydroxyl groups present in an ortho position, while the latter are isolated carboxylic groups. The pK value of 9.8 corresponds to the dissociation of the phenolic groups. The extreme pH values of 2.5 and 10.6, therefore, correspond to situations where either none of the functional groups are dissociated or all groups are dissociated, respectively. Like in earlier reports we, too, find that the fluorescence spectra of GO 3

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Figure 2. (a) TRES and (b) TRANES plot of GO dispersions at pH 5.5 recorded at different emission wavelengths from 360 to 590 nm in 5 nm intervals. The excitation wavelength was 320 nm. The time step of the measurements was 0.4 ns in the delay time interval 5 to 10 ns and 1 ns for the interval 10 to 15 ns.

Scheme 2. Excited State ‘Intra’-molecular Proton Transfer in GO at pH 5.5 Involving Carboxylate Groups and Neighboring Hydroxyls

fluorescence spectra of GO dispersions at pH 10.6 shows a single band at 470 nm for excitation at 300 nm that shifts to 495 nm on exciation at longer wavelengths. A shoulder at 470 nm may still be seen in the emission spectra recorded at 340 nm exciation. The 495 nm band corresponds to the phenolate ion that subsequently transforms to an aromatic ketone (quinone formation).28 It is the latter species that is responsible for the emission at 470 nm seen on excitation at lower wavelengths. The overall picture that emerges from the steady state fluorescence spectra of GO dispersions is that the changes in the fluorescence spectra with pH can be understood in the context of the sequence of dissociation events that occurs as the pH is changed (Scheme 1). The quasi-molecular fragments responsible for the emission at different values of pH may be assigned based on the pK values of the species that dissociate at these values of pH and the reported values of the emission wavelengths of the corresponding molecular species. The results and assignments are summarized in Scheme 1 (see also Supporting Information). To complement the steady-state fluorescence measurements, we have carried out time-resolved measurements. The TRES measurements were made at two excitation wavelengths 320 and 360 nm. These were chosen based on the steady state fluorescence results (Scheme 1). The TRES spectra at different values of pH were constructed from the florescence decay measurements recorded at 5 nm intervals (see Supporting Information) and the corresponding steady state spectra. The TRES data at pH 2.5 is not discussed; the TRES are identical to the steady-state spectra at this pH.

The TRES spectra of GO dispersions at pH 5.5 on excitation at 320 nm are shown in Figure 2a and the corresponding TRANES spectra in Figure 2b. At very short delay times (5 ns), the spectra shows two features: a band at 407 nm that may be assigned to singlet emission from carboxylate groups (pK ∼ 4.3) that are ortho to hydroxyl groups, and a band at 540 nm, assigned to isolated carboxylic groups (pK ∼ 6.6) that are not ionized at pH 5.5. The 540 nm band is comparatively shortlived, and by 8 ns is almost absent. The 407 nm shows a red shift with delay time reaching a value of 430 nm at a delay time of 7 ns and subsequently no further shift with delay. This spectral migration on nanosecond time scale is a consequence of excited-state ‘intra’-molecular proton transferthe hydroxyl proton is transferred to the carboxylate anion in the excited stateas shown in Scheme 2. This behavior is very similar to that observed in the fluorescence spectra of 3-hydroxy napthoic acid that is known to undergo excited state intramolecular proton transfer.26 We have measured the TRES of 3-hydroxy napthoic acid under similar conditions (see Supporting Information) and observe a similar red-shift from 403 nm band to 430 nm in the first 10 ns following excitation. The absence of an iso-emmissive point in the TRANES spectra (Figure 2b) indicates that the 407 and 430 nm emitting species in GO are not in equilibrium. It is interesting to ask what happens when the hydroxyl protons (pK 9.8), too, are ionized. As expected, at pH 10.6 the 407 nm band seen at pH 5.5 is no longer observed; instead the TRES spectra shows a band at 495 nm at very short delay times that shifts to shorter wavelengths (470 nm) with increasing delay (>8 ns) (Figure 3). This shift is not associated with 4

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Figure 3. (a) TRES and (b) TRANES plot of GO dispersions at pH 10.6 recorded at emission wavelengths from 355 nm to 600 nm in 5 nm intervals. The excitation wavelength was 320 nm. The time step of the measurements was 0.4 ns from 6 to 15 ns.

Scheme 3. Transformation of the Phenolate Anion Present on GO at pH 10.6 to the Corresponding Ketone on Photo-excitation

Figure 4. (a) TRES plots of GO dispersion at pH 5.5 recorded at an excitation wavelength of 360 nm. (b) TRES plots and (c) the corresponding TRANES plots of GO dispersion at pH 7.5 recorded at an excitation wavelength of 360 nm. The time-resolved emission spectra were recorded from 430 to 660 nm in intervals of 5 nm. The time step in the TRES measurements was 0.5 ns from 4 to 10 ns.

proton transfer (at this pH all ionizable groups are dissociated) but arises because of the transformation, in the excited state, of the phenolate anion (λem= 495 nm) to an aromatic ketone (quinone formation) (λem = 470 nm) as shown in Scheme 3. The fluorescence emission of the ionized carboxylic groups on GO also show changes with delay time. The TRES spectra of GO dispersions at pH 5.5 and 7.5, on exciation at 360 nm are shown in Figure 4. Changes with delay time are similar at these values of pH but are more clearly seen at the higher pH when all carboxylic groups are ionized. The TRES and TRANES spectra of GO dispersions at pH 7.5 shows two well resolved bands centered at 478 and 515 nm. At short delay times the 478 nm band appears more intense than the 515 nm band, but with longer delay the relative intensities are reversed, and at a delay time of 8 ns the 515 nm band is more intense. It may be recalled that the steady state spectra at this pH (Figure 1e) showed an emission maxima at 515 nm. The TRANES

shows no isoemissive point. A similar behavior is observed at pH 5.5. These changes may be understood from the fact that pK values of carboxylic groups may be quite different in the excited state.29 Such an explanation has been invoked to explain changes in then steady state fluorescence spectra of GO with pH in earlier reports.14 The 478 nm emission may be assigned to isolated carboxylate anions (−COO−)* present on GO, and the decrease in its relative intensity with time is because it under goes excited state protonation by the solvent water molecules (Scheme 4). The 515 nm band that becomes increasingly prominent at longer delay times, is due to emission from undissociated −COOH*. Protonation is known to shift the emission to longer wavelengths. The absence of an isoemmissive point indicates the absence of equilibrium between the excited dissociated and undissociated −COOH* on these time scales, at this pH. 5

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Scheme 4. Excited State ‘Inter’-molecular Proton Transfer from Solvent Water Molecules to Carboxylate Groups Present on GO

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In summary, we have shown that the changes with pH in the steady-state fluorescence spectra of aqueous dispersions of GO correlate well with the sequence of dissociation events of the functional groups present on GO, that had been previously established from pKa measurements and infrared spectroscopy. These events are, with increasing pH, the dissociation of isolated carboxylic groups on the GO sheet that have hydroxyls in close proximity, at the ortho position (pK ∼ 4.3), the dissociation of the isolated carboxylic groups (pK ∼ 6.6,) and finally the dissociation of the phenolic groups (pK ∼ 9). The changes in the fluorescence spectra with pH reflect these events. At pH 2.5 none of the functional groups are ionized and the emission maxima appears at 550 nm while at pH 5.5 carboxylic groups that have hydroxyl groups in close proximity are ionized and the emission maxima now appears at 428 nm (λex = 320 nm). At still higher pH values (pH 7.5) emission from the remaining COO− groups are observed at 515 nm (λex = 360 nm) while in very basic medium (pH 10.5) emission at 470 nm due to the ionized phenolic groups (pK) is observed. These results reconfirm that the fluorescence spectra of GO, at least in part, is quasi-molecular in origin. The time-resolved measurements compliment the steadystate fluorescence data and show that the fluorescence phenomena exhibited by aqueous GO dispersions is much richer than what was initially anticipated. We find that the pH dependent features in the fluorescence spectra, described in the earlier sections, exhibit either spectral migration or changes in relative intensities on the nanosecond time-scale. These changes are a consequence of excited-state proton transfer involving the quasi-molecular fluorophores, and both intramolecular as well as intermolecular, involving solvent water molecules, transfers are observed. The spectral migration from 407 to 430 nm on nanosecond time-scales is associated with intramolecular transfer involving carboxyalte anions on the edges of the GO sheet that are ortho to a hydroxyl group. In the excited state, the hydroxyl proton is transferred to the carboxylate. We show that this behavior is similar to that exhibited by the 3-hydroxy napthoic acid. The increase in relative intensity of the 515 nm band with respect to the 485 nm band in the time-resolved measurements is because of proton transfer from water molecules to carboxylate anions in the excited state. We find that just as in the case of the steadystate fluorescence spectra the changes observed in the timeresolved measurements can be understood in terms of the properties of the quasi-molecular functional groups on GO in their excited state. In conclusion, we have shown here that it is the chemistry associated with quasi-molecular chromophores present on GO that dicates the photophysics of GO sheets in their aqueous dispersions.

ASSOCIATED CONTENT

S Supporting Information *

(S1) Preparation and characterization of graphene oxide. (S2) XPS, 13C CP-MAS NMR, Raman spectra of graphene oxide. (S3) Fluorescence spectra of GO dispersions at pH 2.5 at different excitation wavelengths. (S4) Time-resolved fluorescence decay curves of GO dispersions at pH 5.5 and 10.6 with λex = 320 nm. (S5) Time resolved fluorescence decay curves of GO dispersions at pH 5.5 and 7.5 with λex = 360 nm. (S6) Steady state fluorescence spectra and time-resolved fluorescence decay curves of 3-hydroxy-2-naphthoic acid at pH 5.5. (S7) TRES and TRANES plots of 3-hydroxy-2-naphthoic acid at pH 5.5. This material is avilable free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. Notes

The authors declare no competing financial interest.

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REFERENCES

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