With the occurrence of heavy metals in blood, drinking water, and growing environmental pollution, great attention has been paid to heavy metal ion detection with high sensitivity and selectivity [1].
Photonics Research, Volume. 11, Issue 10, 1781(2023)
Plasmon-enhanced fluorescence of gold nanoparticle/graphene quantum dots for detection of Cr3+ ions
Graphene quantum dots (GQDs), fascinating semiconductors with stable photoluminescence (PL), have important potential applications in the fields of biology, medicine, and new semiconductor devices. However, it is still challenging to overcome the weak PL intensity. Here, we report a strategy for selective resonance enhancement of GQD fluorescence using gold nanoparticles (AuNPs) as plasmas. Interestingly, the addition of low concentration AuNP makes AuNP/GQDs exhibit significant fluorescence enhancement of 2.67 times in the visible range. The addition of high concentration AuNP leads to the formation of an excitation peak at 421 nm and selectively enhances certain radiation modes. We concluded that the main reason for the selective enhancement of PL intensity in high concentration AuNP is the transfer of generous hot electrons at high energy states from AuNP to GQD and relaxation to the ground state. The electron resonance of low concentration AuNP transfers to GQD and relaxes to lower energy levels, exhibiting an overall enhancement of PL intensity. We apply it for detection of the heavy metal ion
1. INTRODUCTION
With the occurrence of heavy metals in blood, drinking water, and growing environmental pollution, great attention has been paid to heavy metal ion detection with high sensitivity and selectivity [1].
As 0D material, graphene quantum dots (GQDs) have aroused great interest in research and practical applications due to their superior and exciting chemical, physical, mechanical, and electronic properties [7]. They have open bandwidths due to quantum confinement, excellent dispersibility, and more abundant active sites [8–10]. GQDs have the excellent performance required for biomedical applications such as biocompatibility, which gives them unique advantages in heavy metal sensing. The enhanced photoluminescence (PL) emission of GQDs can improve their performance in photoelectric detectors and minimize the effect of different noises on the efficiency of such devices, resulting in higher sensitivity [11,12].
The PL intensity of a fluorescent molecule can be strongly enhanced when near a metal nanoparticle (NP) [13,14]. Localized surface plasmon (LSP) resonance enhanced PL has gained considerable attention in numerous applications and research areas, such as biology and medical science [15–18]. It is necessary to understand the mechanism for such a good effect if we intend to develop better materials to improve the enhancement. Usually, PL enhancement can be attributed to two means: a localized enhanced electric field [19] and plasmon resonance energy transfer [20]. A localized enhanced electric field refers to the local electric field generated by photoexcited plasmon crystals, and the PL emission of fluorescent molecules increases under a strong local electric field [21,22]. Plasmon resonance energy transfer is similar mechanism [23,24]. The process can be described as follows: the plasmon crystals absorb photons and excite plasmon resonance, and energy is transferred to fluorescent molecules through intraband excitations within the conduction band or through interband excitations caused by transitions between other bands (for example, D bands) and the conduction band. The donor transfers rich hot carriers on the surface of the acceptor molecule through the dipole–dipole coupling of non-radiation. However, the mechanism of PL enhancement is too complex to distinguish which mechanism plays a major role [25].
In our work, GQDs were attached to the surface of a gold NP (AuNP) to enhance PL intensity selectively. To investigate whether the localized enhanced electric field or plasmon resonance energy transfer is the main mechanism of PL enhancement, we conducted a control experiment to isolate the adiabatic electron transfer path and adjust the AuNP near field strength. It proved that AuNP plays the role of a dipolar antenna, mainly by collecting incoming photons from a range much larger than GQDs’ and transferring hot electrons to the electron transition path of electron–hole recombination in the GQDs’ electronic structure. Density functional theory (DFT), Raman spectrum, time-resolved PL (TRPL) decays, and the PL spectrum proved that the reason for selective fluorescence enhancement or overall enhancement of graphene is that due to the addition of different concentrations of AuNP, the electrons directly relax from the high energy band to the ground state or a lower energy level and then return to the ground state.
2. RESULTS AND DISCUSSION
AuNPs were prepared by a modified seed-mediated method. To verify the crystal nature of the AuNP/GQDs, Figs. 1(a) and 1(b) show the representative transmission electron microscopy (TEM) and high angle annular dark-field-scanning TEM (HADDF-STEM) images of AuNP/GQDs. The diameter of AuNP coated randomly with GQDs was
Figure 1.(a) Representative transmission electron microscopy (TEM) and (b) high angle annular dark-field-scanning transmission electron microscopy (HADDF-STEM) images of the AuNP/GQDs. (c) High resolution transmission electron microscopy (HRTEM) image of the AuNP/GQDs; the red circle inside is GQD, inset is a scaled up view of GQD with high resolution, lattice spacing is 0.21 nm, and crystal plane is (100). (d) Selected area electron diffraction (SAED) pattern of AuNP/GQDs.
Figure 2.(a) Representative transmission electron microscopy (TEM) and (b) high resolution transmission electron microscopy (HRTEM) images in the red region of AuNPs. (c) Statistical diameter of AuNPs.
X-ray photoelectron spectroscopy (XPS) confirmed the presence of electron donors and electron acceptors. Figure 3 shows XPS spectra containing GQD and AuNP/GQDs in which AuNP/GQDs = 400:4. Deconvolution of the C 1s spectrum [Fig. 3(a)] resulted in the observation of C═C, C═O, and ─COOH bonds with binding energies of 284.6 eV, 286.7 eV, and 288.4 eV, respectively [26,27]. Deconvolution of the O 1s spectrum [Fig. 3(b)] resulted in the observation of C═O and C─O bonds with binding energies of 531.6 eV and 533.2 eV, respectively [26,27]. The ratio of the different groups in GQDs and AuNP/GQDs is shown in Fig. 3(c). XPS results indicate that a relative proportion of the electron donor group (─OH) and electron acceptor carboxy ─COOH in AuNP/GQDs has no significant changes relative to GQDs. In addition, the peak in Fig. 3(d) is attributed to the
Figure 3.(a) Binding energies correspond to the C 1s of GQDs and (b) O 1s of AuNP/GQDs. (c) Ratio of different groups in GQDs and AuNP/GQDs. (d)
We systematically studied the spectral properties of the materials. Figure 4(a) shows the ultraviolet-visible (UV-Vis) absorption spectra of AuNP/GQDs and GQDs. A typical absorption peak at 300 nm along with an extension that proceeds to the visible region was observed, which was inferred to the
Figure 4.(a) Ultraviolet-visible (UV-Vis) absorption spectra. (b) Excitation wavelength-dependent photoluminescence spectra with monitoring wavelength of 550 nm. Photoluminescence spectra of GQDs, GQDs:AuNP = 400:1, 400:2, 400:3, 400:4 excited by (c) 300 nm, (d) 325 nm, (e) 368 nm, and (f) 421 nm.
TRPL measurements of AuNP/GQDs were performed to clearly investigate the impact of AuNP inclusion on GQDs. The corresponding normalized PL decay curves of GQDs and AuNP/GQDs are provided in Fig. 5(a). Based on previous studies, we fitted the decay curves by a multiexponential function with deconvolution of the instrument response function (IRF):
Figure 5.(a) Photoluminescence lifetime of GQDs, GQDs:AuNP = 400:1 and 400:3. (b) Extracted photoluminescence lifetime parameters of GQDs and AuNP/GQDs.
When the metal NP is excited by light of a specific wavelength, the conduction electrons of the metal will oscillate collectively (i.e., plasma). The strong electromagnetic field associated with LSP resonance can be confined to the deep subwavelength space near the particle surface. Near-field enhancement of an electromagnetic field and hot electron injection are considered two potential reasons for PL enhancement. The surface of
Figure 6.(a), (b) TEM images of AuNPs coated with silicon oxide. (c) Excitation wavelength-dependent photoluminescence spectra and (d) photoluminescence spectra of GQDs and AuNP/GQDs coated with silicon oxide of different thicknesses, with excitation wavelength of 325 nm. Calculated electric field of (e) AuNP, (f)
To further illustrate the specific electron transfer reaction of AuNP, the band structure of the Au was calculated based on DFT [53,54], as shown in Fig. 7(a). The projected density of states (PDOS) [Fig. 7(b)] of AuNP was obtained from Fig. 7(a). There were two broad regions in PDOS, where a valence band appeared below
Figure 7.(a) Calculated electronic band structure and (b) projected density of states (PDOS) of Au. (c) Band diagram of AuNP/GQDs.
Pollution by heavy metal ions is a serious environmental problem. Heavy metal ion toxicity has been reported to cause many health issues to live beings and has motivated researchers to develop various strategies for the detection and removal of these heavy metal ions from aqueous systems to make water safe for use. In our work, AuNP/GQDs can be applied as a fluorescent probe for the assay of
Figure 8.(a) Fluorescence spectra of AuNP/GQDs for different heavy metal ions. (b) Different heavy metal ion detection effect statistics of AuNP/GQDs at a concentration of 50 μM. (c) Fluorescence spectra for detection of different concentrations of chromium ions. (d) Correlation coefficient of AuNP/GQDs and different concentrations of chromium ions; blue area corresponds to poison concentrations.
3. EXPERIMENT
A. Reagents
Sodium hydroxide (98%) and L-ascorbic acid (AA, 99.99%) were purchased from Macklin. Hexadecyltrimethylammonium bromide (CTAB, 99.0%) and GQDs were purchased from Aladdin. Tetraethyl orthosilicate (TEOS, 28.4%), chloroauric acid (99.99%), sodium borohydride (96.0%), and hydrochloric acid (36%−38%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
B. Synthesis of AuNP/GQDs
AuNPs were prepared by a modified seed-mediated method. Typically, 0.5 mL of
C. Synthesis of
To synthesize the
D. Fluorescence Assay of
Specifically, 1 mL AuNP/GQDs solution was put into 8 mL of ultrapure water, followed by adding different amounts of
E. Sample Characterization
TEM, HRTEM, and HADDF-STEM images and the SAED pattern were obtained using an FEI Tecnai G2 F20 X-Twin microscope operated at an accelerating voltage of 200 kV. The UV-Vis absorption spectra were measured by a Thermo Scientific Evolution 220 UV-Vis instrument, while the PL spectra were measured by a Hitachi F-460, and PL decay curves were assessed by an Edinburgh FLS1000 spectrophotometer under an excitation wavelength of 365 nm. In addition, the XPS spectra were measured using a Thermo Scientific K-Alpha+. The SERS spectra were acquired with a 514 nm wavelength laser source using a Horiba LabRAM HR Evolution. We put the sample into the capillary tube, collected three ranges in different areas, and took the average. FTIR spectroscopy absorption spectra were measured by a Thermo Scientific Evolution NICOLET iS50.
F. Computational Details
In this work, all DFT calculations were performed using generalized gradient approximation (GGA) and the Perdew–Burke–Ernzerh (PBE) exchange-correlation function. The employed periodic cell dimension was
4. CONCLUSION
In conclusion, the plasmon resonance caused by the AuNP as an antenna can considerably boost light absorption larger than GQDs and thus enhance the PL emission of GQDs. The control experiment of isolating the adiabatic electron transfer path and adjusting the near-field strength proves that the energy transfer caused by plasmon resonance in AuNP/GQDs is the main reason for the PL enhancement of AuNP to GQDs. Furthermore, we can selectively enhance the fluorescence of GQDs by controlling the addition of AuNP. AuNP/GQDs show great potential in the detection of
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You-Long Chen, Yi-Hua Hu, Xing Yang, You-Lin Gu, Xin-Yu Wang, Yu-Hao Xia, Xin-Yuan Zhang, Yu-Shuang Zhang, "Plasmon-enhanced fluorescence of gold nanoparticle/graphene quantum dots for detection of Cr3+ ions," Photonics Res. 11, 1781 (2023)
Category: Optical and Photonic Materials
Received: May. 18, 2023
Accepted: Jul. 25, 2023
Published Online: Oct. 7, 2023
The Author Email: Yi-Hua Hu (skl_hyh@163.com), Yu-Shuang Zhang (yszhang@hnu.edu.cn)