Triphenylphosphine modified graphene quantum dots: spectral modulation for full spectrum of visible light with high quantum yield

Abstract

We report triphenylphosphine (P(Ph)₃) modified graphene quantum dots (P-GQDs) with high quantum yield (c) and excellent stability. The light energy absorbed by P(Ph)₃ groups can quickly and efficiently transfer to the GQDs through the C–P-(Ph)₃ bonds, resulting in a high φ. The obtained P-GQDs have a tunable photoluminescence wavelength from blue to red, and at the same time maintain a high φ, over 58%, with increase of permittivity of the solvents. The main mechanism of full-visible-light-spectrum modulation could be due to the energy loss and electron-donating decrease caused by P(Ph)₃ group–solvent interaction or relaxation.

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Graphene quantum dots (GQDs) are a rising star in the family of fluorescent materials.^1–5 GQDs have superior properties, such as high photostability, biocompatibility and low toxicity compared with organic dyes^6,7 and semiconductor quantum dots.^8–14 Moreover, GQDs clearly contain a 2D graphene structure regardless of the dot size and shape, which is different to hydrothermally synthesized carbon dots.^15–17 However, there are still some demanding challenges in practical applications of GQDs, such as low quantum yield (φ) and controllable spectral modulation.^1–3,18 The low φ means low fluorescent intensity. So far, most reported GQDs just have a φ lower than 0.5,^19,20 and almost all reported GQDs absorb and emit relatively short wavelength light (maximum emission wavelengths λ_em < 500 nm), as listed in detail listed in Table S1.† The GQDs with short emission wavelength have poor tissue permeability, and thus may cause photodamage to tissue and show interference by biological self fluorescence.^2–4,18,21 It is clear that both the low φ and short λ_em restrict the applications of GQDs in fluorescent bio-imaging, light harvesters, sensitizers for photodynamic therapy and solar cells.

To date, mainstream technologies for spectral modulation and photoluminescence enhancement of GQDs are based on doping and modification (Table S1†). We reported 1–3 nm nitrogen-doped GQDs with a highest quantum yield of 0.74 via cutting a graphene oxide precursor and an in situ lattice doping process without further surface passivation or modification.¹⁸ Lee et al. reported the synthesis of near infra-red GQDs and their biological application for optical imaging of deep tissues and organs.²² However, there are few reports on the synthesis of GQDs with both tunable spectrum and high quantum yield at the same time. It is well known that GQDs have excitation wavelength dependent emission, i.e., the λ_em varies with different excitation wavelength. Generally, the quantum yield with most excitation wavelengths is very low.^18–22

Here, we report triphenylphosphine (P(Ph)₃) modified GQDs (P-GQDs) with high quantum yield, over 58%, among the full visible light range from blue to red. The P(Ph)₃ groups on P-GQDs absorb light energy, which can quickly and efficiently transfer to the GQD fluorophores through C–P-(Ph)₃ bonds, resulting in a high quantum yield. The λ_em of P-GQDs red-shifted from 431 to 641 nm with the increase of dielectric constant (ε) of the solvent. A spectral modulation mechanism is proposed and discussed.

The P-GQDs were prepared as follows: typically, 2.0 mL 100 mM triphenylphosphine DMF solution was added to 8.0 mL, 1 mg mL⁻¹ graphene oxide quantum dots (GOQDs) DMF solution, and then the mixture was transferred into a 15 mL Teflon®-lined autoclave and heated and kept at 260 °C for 72 h. After that, the mixture was filtered using an alumina inorganic membrane with 20 nm pores. The resulting filtrate was diluted with water and dialysed in a 500 Da dialysis bag against deionised water for a week to remove excess salt. The P-GQD yield from GOQDs is approximately 61%.

As shown in Fig. 1a, the P-GQDs exhibited excellent uniformity. The average diameter of P-GQDs is 3.97 and the size distribution accords well with Gaussian distribution. The full-width at-half-maximum (FWHM) of the fitted Gaussian curve is 1.42 nm, indicating the narrow distribution nature of the P-GQDs. The size distribution exhibits no obvious change after doping (Fig. S1†). The high-resolution TEM image, as shown in the inset of Fig. 1a reveals the high crystallinity of the P-GQDs, showing crystallinity with lattices of 0.226 nm.^2,18,23 This can be attributed to the (002) facet of graphitic carbon.¹⁸

Fig. 1 (a) TEM image and corresponding size distribution histogram of P-GQDs, the black line is the Gaussian fitting curve. Inset: RTEM image of a single P-GQDs particle. (b) UV-vis absorption spectrum of P-GQDs aqueous solution.

The UV-vis spectrum of P-GQD solution (Fig. 1b) shows an absorption peak at 209 nm, which is ascribed to the π–π* transition of C=C in the conjugated structure. The significant absorption peak at ca. 250 nm reveals the n–π* transition of the p–π orbit between the P and conjugate structure.^24,25 The XPS survey spectrum of P-GQDs is shown in Fig. 2a, showing a narrow graphitic C 1s peak at ca. 284.2 eV, a O 1s peak at ca. 532 eV and a P 2p peak at ca. 126 eV. Fig. 1b shows the well-fitted C 1s spectrum, which can be divided into four different peaks, corresponding to the signals of C–C and C=C (284.6 eV), C–OH (285.4 eV), –COOH (289.1 eV) and C–P (281.6 eV).^24–26 The P 2p spectrum can be divided into two different peaks (Fig. 1c), corresponding to the signals of P–C (127.3 eV) and P–Ph (128.2 eV). Moreover, the O 1s spectrum also can be divided into two different peaks (Fig. 2d), which correspond to the signals of C–O (533.5 eV) and C=O (531.0 eV).²⁷ Thus, the P(Ph)₃ have been successfully used to modify the GQD via the C–P-(Ph)₃ bonding mode.²⁵ The FT-IR spectrum of P-GQDs is shown in Fig. S2.† The adsorption peaks at 3500–2500 cm⁻¹ correspond to the –OH stretching mode, which implies the presence of –OH and –COOH. The peak at 1790 cm⁻¹ can be attributed to the stretching vibration of C=O. The strong absorption peak at 1150 cm⁻¹ can be due to the stretching vibration of P–Ar groups. The peak at 950 cm⁻¹ can be attributed to the stretching vibration of C–O–C. Moreover, the bands at 754 and 593 cm⁻¹ are characteristic of C–H out-of-plane bending vibrations of benzene nuclei in the benzene ring.²⁹

Fig. 2 (a) XPS survey spectrum of P-GQDs. (b) C 1s XPS spectrum, (c) P 2p XPS spectrum, (d) O 1s XPS spectrum of P-GQDs.

The optical properties are shown in Fig. 3. The P-GQDs DMF solution exhibited maximum excitation (λ_ex) and emission (λ_em) wavelengths at 460 and 532 nm, respectively (Fig. 3a). The optical images indicate that the P-GQDs give bright blue-green photoluminescence (PL) under UV light (Fig. 3a inset). The Stokes shift (Δν_St) and FWHM of P-GQDs are 0.36 eV and 0.24 eV, respectively. The low Δν_St indicates that the P-GQDs have a weak self-absorption effect and low energy loss. The φ of P-GQDs is 0.64, which is higher than most previous reports (Table S1†).^1–6,18,28

Fig. 3 (a) Normalized PLE (black curve) and PL (red curve) spectra of P-GQDs DMF solution. Inset: digital image of P-GQDs DMF solution under UV-light (centre wavelength: 365 nm). (b) PL decay curves of P-GQDs measured at room temperature and excitations at 460 nm. (c) PL spectra of P-GQDs with different excitation wavelength (410, 420, 430, 440, 450, 460, 470, 480, 490, 500 nm). (d) Photostability (red curve) of P-GQDs under UV light (150 W Xe lamp with centre wavelength at 320 nm). F₀ and F are PL intensities at 0 h and a specified time, respectively.

The PL decay of P-GQDs is measured by a time-correlated single photon counting technique, and fitted well with a bi-exponential decay as shown in Fig. 3b. The lifetime (τ) is dominated by a long decay component of 7.1 ns (88%) plus a small contribution from the short decay of 1.3 ns (12%): the weighted-average lifetime is approximately 6.4 ns. However, the physics of fast and slow decay in GQDs is unclear, which could be due to the complex PL mechanism and diverse surface activity groups. Combining the φ and τ by κ_r = φ/τ, we can obtain the radiative rate κ_r.²⁴ The fluorescence radiative rate κ_r of P-GQDs is 1.0 × 10⁸ s⁻¹.^30–32

The P-GQDs exhibit a maximum emission wavelength shift of 26 nm when the excitation wavelength increases from 410 to 500 nm. Moreover, the P-GQDs showed excellent photostability under UV light (Fig. 3d). The PL intensity has no obvious change under a 150 W Xe lamp with a centre wavelength of 320 nm for 24 h. Moreover, the PL intensity of P-GQDs is also stable when the pH value was changed from 1 to 14 (Fig. S3†). The good stability is due to the pH insensitive groups P-(Ph)₃ on P-GQDs.

It is interesting that the λ_em shifted when the P-GQDs were dispersed in different solvents. Fig. 4a shows the normalized PL spectra of P-GQDs in different solvents (ethyl acetate (EA), acetone (DMK), ethanol (EtOH), DMF, nitromethane, DMSO and water). The λ_em shifted from 431 to 641 nm. The tunable optical spectrum almost covered the full spectrum of visible light (400–700 nm). The P-GQDs gave colorful bright luminescence from blue to red when the P-GQDs were dispersed in different solvents (Fig. 4b). Moreover, the quantum yield always remained 0.58 or higher (red curve in Fig. 4c), which indicates that the P-GQDs have good PL properties in different solution environments. Further analyses show that the λ_em is related to the permittivity (ε) of the solvents (Fig. 4c). With the increasing ε, the λ_em red-shifted.

Fig. 4 (a) PL spectra of P-GQDs in different solvents. (b) Digital image of P-GQDs in different solvents under UV-light (centre wavelength: 365 nm). (c) The relation between λ_em, φ and ε. (d) Schematic diagram illustrating the possible mechanism of the modulation progress.

The possible mechanism of the spectral modulation is shown in Fig. 4d. As a large 2D conjugated molecular structure, GQDs can be regarded as a macromolecule. The photoluminescence process of GQDs is much more like that of fluorescent dyes. P-GQDs possess electron-donating (P(Ph)₃) groups conjugated to an electron-withdrawing 2D macromolecule (GQDs). The P(Ph)₃ groups have high UV absorption cross sections,^33,34 and the harvested energy can quickly and efficiently transfer to the GQD fluorophores through the C–P-(Ph)₃ bonds. This energy transfer process affords high quantum yield, and this PL enhancement mechanism is similar to the n–π* transition between the N in the aromatic ring and the conjugate structure of graphene in N-doped GQDs, with very high quantum yield.¹⁸ On the other hand, the P(Ph)₃ groups will be affected by the solvent due to their spatial pyramidal structure with a chiral propeller-like arrangement of the three phenyl rings. When the solvent polarity increases, the P(Ph)₃–solvent interaction/relaxation may lead to energy loss and a decrease of the electron-donating capacity, which contributes to the red shift of the fluorescence spectra, as shown in Fig. 4b and c. It should be mentioned that the solvent polarity has less impact on GQDs since it has a much larger 2D structure compared with P(Ph)₃. The energy diagram of P-GQDs with different ε is proposed in Fig. S4† for a better understanding of the mechanisms.

In summary, we fabricated P-GQDs with a tunable spectrum from 431 to 641 nm, a high quantum yield over 0.58 and excellent stability. The P(Ph)₃ was connected to GQDs through chemical C–P-(Ph)₃ bonds. The P(Ph)₃ groups absorb light energy and transfer energy to the GQDs, resulting in a high quantum yield. The energy loss and electron-donating capacity reduction due to the P(Ph)₃–solvent interaction/relaxation may contribute to the spectral modulation. This work will be helpful for synthesizing new bio-imaging materials with both high quantum yield and tunable PL properties at the same time.

Acknowledgements

This work was supported by projects from the National Science and Technology Major Project (Grant no. 2011ZX02707), the Chinese Academy of Sciences (Grant no. KGZD-EW-303 and XDA02040000).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04001a

‡ These authors are co-first-authors.

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