Ultrasensitive Detection of Fe3+ Ions Using Functionalized Graphene Quantum Dots Fabricated by a One-Step Pulsed Laser Ablation Process.

With respect to the detection of Fe3+ ions, graphene quantum dots (GQDs) have limitations for commercialization owing to their high limit of detection (LOD). Here, we report a one-step pulsed laser ablation (PLA) process to fabricate amino-functionalized GQDs (FGQDs) for the efficient detection of Fe3+ using polypyrrole (PPy) both as a precursor (amine N) and as a surfactant and also using graphite as a carbon precursor. Using this method, the amine N groups were easily incorporated into the carbon network of the GQDs. Additionally, compared to pristine GQDs, FGQDs showed smaller particle sizes and narrower size distributions owing to the surface passivation effects of the PPy surfactant. Due to the synergistic effect of surface passivation and incorporation of amine N groups, FGQDs exhibited a sensitive response to Fe3+ ions in the concentration range of 500 nM to 50 μM, which is lower than the quality standards for Fe3+ ions (∼5.36 μM) as suggested by the World Health Organization (WHO). Furthermore, the processing time for synthesizing FGQDs by the PLA process was less than 30 min, thus allowing successful practical applications of GQDs in the sensing field.

Introduction

The global issue of heavy-metal-ion pollution has attracted significant attention in the past few decades because of critical problems such as water pollution and human health.[1,2] Heavy-metal ions (e.g., Cd2+, Fe3+, Cu2+, Co2+, Ni2+, Al3+, and Ag3+) can be found in water, and eventually turn more toxic inside the human body. These heavy-metal ions are nondegradable in nature, leading to their continuous accumulation in the soil and human body. Therefore, several studies have been undertaken for the detection of heavy metals to date.[3−6]

Among these heavy-metal ions, Fe3+-ion sensing is important because the deficiency or abundance of Fe3+ can cause serious diseases, such as anemia, Parkinson’s syndrome, Alzheimer’s, and even cancer.[7,8] Several fluorescence probes such as organic fluorescent dyes, polymer nanodots, and semiconductor quantum dots have been widely studied for the detection of Fe3+ ions.[9−12] However, they suffer from certain disadvantages such as poor water solubility, low photostability, and high toxicity, which limits their reliable application in real sample assays.

Recently, graphene quantum dots (GQDs) have emerged as a potential new platform as fluorescent probes for the detection of Fe3+ ions because of their low toxicity, high water solubility, and photostability.[13−17] However, the GQDs exhibit a low limit of detection (LOD) for the Fe3+ ions owing to the limited number of surface active sites.[18−20] To overcome these issues, several studies have been performed to enhance the active sites on the GQDs. For instance, functionalized GQDs with amino groups (e.g., amine N) provide abundant active sites, which lead to superior detection performance of Fe3+ ions.[21] In addition, narrower size distribution and smaller size of GQDs were obtained by the effect of surface passivation using a surfactant (i.e., PPy), resulting in an increased specific surface area.[22] Owing to the large number of active sites due to the increasing specific surface area of the GQDs, the detection performance for Fe3+ ions was enhanced. Consequently, the effect of surface passivation and functionalization of GQDs with amine N groups can lead to an improvement in the active sites, which may play a key role in the enhancement of the Fe3+ detection limit.[23]

Recently, several studies have reported the synthesis of FGQDs by chemical oxidation, electrochemical preparation, and hydrothermal methods.[24−26] However, these methods are usually performed under strongly acidic conditions, which require a prolonged washing step. Furthermore, for the surface passivation and functionalization of GQDs, multiple synthesis processes such as heat treatment and deep coating are required, which are costly and time-consuming.[27,28] Therefore, it is necessary to develop a one-step synthetic method.

Pulsed laser ablation (PLA) is an alternative method for the synthesis of FGQDs. Previous studies have reported the successful preparation of functionalized GQDs with the incorporation of heteroatoms (e.g., N, S) via a one-step PLA process.[29−33] Additionally, lauryl dimethylaminoacetic acid betaine (LDA) was employed as a surfactant for the surface passivation of Tb3Al5O12:Ce3+ nanoparticles during the PLA process, which resulted in narrower size distribution and smaller size of nanoparticles.[34] In addition, compared to chemical methods, the PLA process is simple and chemically clean because it does not require postprocessing steps.[35] Therefore, the PLA is a potential method for the synthesis of FGQDs.

Here, we report a one-step strategy to fabricate FGQDs by PLA process using PPy both as a precursor (i.e., amine N) and surfactant. The effect of surface passivation using PPy during the PLA process enables narrower size distribution and smaller size of the FGQDs. X-ray photoelectron spectroscopy (XPS) revealed that functionalized GQDs doped with amine N can be achieved by the addition of PPy. In addition to these features of the prepared FGQDs, we have also demonstrated highly sensitive and selective fluorescence of the FGQDs for the detection of Fe3+ ions.

Results and Discussion

Figure shows the transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of the pristine GQDs (GQDs, Figure a,e), FGQDs with 0.5 mol % PPy (F0.5GQDs, Figure b,f), 1.0 mol % PPy (F1.0GQDs, Figure c,g), and 1.5 mol % PPy (F1.5GQDs, Figure d,h). Compared to GQDs, the FGQDs exhibited narrower size distributions and smaller sizes, as shown in Figures a–d and S1. The size distribution of the GQDs and the three types of FGQDs were fitted by a Gaussian curve with a 95% confidence interval. Average diameters of 5.0 ± 1.5 and 3.5 ± 0.5 nm were obtained for the GQDs and the three types of FGQDs, respectively, by counting more than 200 of the GQDs. This indicates that the particle size distribution of the GQDs can be controlled by the addition of PPy as a surfactant. Figure e–h shows the HR-TEM images of the GQDs and the three types of FGQDs. According to the fast Fourier transform (FFT) patterns (left insets, Figure e–h), it is clear that the GQDs and three types of FGQDs are crystallized in the pristine graphene structure. In addition, the right insets of Figure e–h show a lattice spacing of approximately 0.24 nm, which is in good agreement with the (1120) lattice planes of graphene.[24,25] The atomic force microscopy (AFM) results are shown in Figure S2. Statistical analysis revealed that more than 93% of the GQDs and three types of FGQDs have a thickness of less than 1.9 nm, corresponding to three to four graphene layers. This result indicates that the pristine GQDs and three types of FGQDs have a few layers, which is similar to that reported in previous studies.[24,29]

Figure 1

TEM images of GQDs (a), F0.5-GQDs (b), F1.0-GQDs (c), and F1.5-GQDs (d). HR-TEM images of GQDs (e), F0.5-GQDs (f), F1.0-GQDs (g), and F1.5-GQDs (h). Corresponding fast Fourier transformation (FFT) patterns (left insets). Lattice spacing of GQDs and the three types of FGQDs (right insets)

We investigated the chemical composition of the three types of FGQDs using X-ray photoelectron spectroscopy (XPS). Figure a shows the C 1s peaks of the three types of FGQDs. The carbon peak can be deconvoluted into four peaks centered at 284.5, 285.7, 286.3, and 287.7 eV, which represent C–C/C=C, hydroxyl, C–N/C=N, and carboxyl groups, respectively. The N 1s spectra of the three types of FGQDs exhibited three types of configurations for C–N bonding (pyridinic, pyrrolic, and graphitic), as shown in Figure b.[31] In addition, we confirmed that the amine N peak located at 399 eV was present in the XPS profiles of the three types of FGQDs.[36] Furthermore, it was observed that as the PPy concentration was increased from 0.5 to 1.5 mol %, the C–N/C=N and amine-N concentrations also gradually increased. In other words, the above-mentioned functional groups (i.e., C–N, amine-N) in the carbon structure are dependent on the PPy concentration, which indicates that the tuning of the chemical composition is controllable in FGQDs.

Figure 2

XPS profiles of the FGQDs (a) C1s and (b) N1s peaks.

A possible mechanism for the transformation of graphite flakes to the FGQDs is shown in Figure . Generally, when a pulse laser is injected into a target (i.e., graphite flakes), extremely harsh environments, such as high temperature and high pressure, are formed owing to multiphoton absorption ionization.[35] Subsequently, the plasma plume and cavitation bubble occur in the synthesis area during the PLA process, which leads to partial decomposition of the graphite flakes, ethanol, and PPy, resulting in C, H, O, and N precursors.[31] Finally, the formation of each precursor (C, H, O, and N) occurs in the synthesis area because of their high surface energy, resulting in the formation of FGQDs. In addition, the aggregation of FGQDs may be suppressed by the addition of PPy owing to the effect of surface passivation. Generally, PPy has a positive charge, whereas FGQDs possess negative polarity because of the abundance of oxygen-rich functional groups, such as hydroxyl and carboxyl groups, on the surface of the FGQDs.[37,38] Thus, the PPy surrounds the FGQDs and forms passivation layers, as shown in Figure . It is noted that the surface of FGQDs may be locally passivated by PPy because of the ultrafast pressure and temperature changes during the PLA process, which may not be sufficiently passivated. These layers can suppress the aggregation of GQDs, resulting in a narrower particle size distribution and smaller size. These results are in good agreement with the HR-TEM images shown in Figure .

Figure 3

Schematic illustration of the possible mechanism for the transformation of graphite flakes to FGQDs by the PLA process.

Figure 4

Schematic diagram showing the surface passivation of the PPy/GQDs

The optical properties of the GQDs and three types of FGQDs were investigated by photoluminescence (PL), UV–visible (UV–vis), and photoluminescence excitation (PLE) spectroscopy. Figure a shows the PL properties of the GQDs and three types of FGQDs. The three types of FGQDs exhibited stronger PL intensities than that of the GQDs, with quantum yields (QYs) calculated at 0.8, 1.84, 2.34, and 1.52% for the GQDs, F0.5GQDs, F1.0GQDs, and F1.5GQDs, respectively. The PL spectra showed that the peak intensity gradually increased with an increase in the PPy concentration up to 1.0 mol %. However, the peak intensity and QY decreased as the PPy concentration exceeded 1.0 mol %. This phenomenon may be occurring owing to the excessive heteroatoms (i.e., N in this work) blocking the passivated surface defects.[39] In addition, the three types of FGQDs showed excitation-dependent PL spectra similar to that reported in previous studies (Figure S3).[32,40] The PLE spectra of the three types of FGQDs are different from that of the GQDs, as shown in Figure b. The PLE spectra of FGQDs exhibited two clear peaks at 260 and 360 nm, which implies that the N-related group (e.g., amine N) leads to an enhanced electron density in the intrinsic state of GQDs.[31] The UV–vis spectra exhibit absorption bands at 210, 260, and ∼360 nm, as shown in Figure c. Typically, the UV–vis absorption bands at 210 and 260 nm are assigned to the π–π* transition of C–C/C=C of the sp2 domain in the carbon structure, and the shoulder peak at ∼360 nm corresponds to the n–π* transition of C=O. Compared to GQDs, the three types of FGQDs exhibited stronger UV–vis peaks at 210 and 260 nm (π–π* transition), indicating the high density of the carbon framework formed. In addition, the absorption site at ∼360 nm in the oxygen-containing groups also increased after the incorporation of amine N, which may be related to the n−π* transition of C=O and also to the amine N group. Thus, the strong electron affinity and superior optical properties of the FGQDs could be useful for the development of metal-ion sensors.[19,20]

Figure 5

Optical properties of GQDs, F0.5-GQDs, F1.0-GQDs, and F1.5-GQDs. (a) PL properties under 360 nm excitation, (b) PLE spectrum, and (c) UV–vis spectrum.

To explore the fluorescence sensing application of FGQDs, we performed PL analysis in the presence of various biologically and environmentally relevant metal ions (50 μM), such as Ni2+, Fe3+, K+, Al3+, Zn+2, Mn2+, Ag+, Cd2+, Ca2+, and Mg2+. The fluorescence intensity ratio of the FGQDs was analyzed by adding various metal ions. According to Figure a, the PL intensity of F1.0GQDs showed the strongest quenching effect in the presence of Fe3+ ions, whereas negligible PL quenching was observed in the presence of the other metal ions. Compared to the other cations, the electron-deficient Fe3+ ions have a higher binding affinity toward electron-rich groups and can easily interact with electron-donating groups such as amino groups (e.g., amine N) on the surface of the FGQDs.[41] Additionally, the FGQDs showed narrower size distributions and smaller particle sizes by surface passivation, which provided abundant active sites.[21,36] Consequently, the adsorption of Fe3+ ions onto the FGQDs was accelerated by both surface passivation and functionalization effects, resulting in high fluorescence quenching. The concentration-dependent fluorescence of F1.0GQDs with different concentrations of Fe3+ ions was investigated for the detection of the Fe3+ ions (Figure b). The PL intensity of the F1.0GQDs decreased as the concentration of Fe3+ ions increased from 500 nM to 50 μM. Compared to the GQDs and N-doped GQDs (without amine N groups; see the Experimental Section), the F1.0-GQDs showed more sensitivity toward the detection of the Fe3+ ions at a concentration of 5 μM, as shown in Figure c. Although the N concentrations of the N-doped GQDs (4.1%) and F1.0GQDs (3.8%) were similar, the limit of detection (LOD) was significantly reduced by the incorporation of the amine N group (Figures , 6c, and S4 and S5). The LOD of the FGQDs is much lower than the water quality standards (i.e., ∼5.3 μM) for Fe3+ ions suggested by the World Health Organization (WHO). The Fe3+ sensing properties of the FGQDs were compared with those of the recently reported GQDs, as shown in Table . The FGQDs prepared in this study showed superior Fe3+ detection performance and ultrafast synthesis process compared to those of GQDs (Table ). Therefore, the FGQDs synthesized by the PLA process with PPy could be suitable sensing materials with high sensitivity and selectivity for the detection of Fe3+ ions.

Figure 6

(a) Fluorescence intensity of aqueous F1.0-GQDs solution in the presence of 50 μM concentration of the different metal ions at λex = 360 nm. F0 and F are the fluorescence intensities of the F1.0-GQDs before and after interaction with Fe3+ ions, respectively. (Insets: Photographs of the aqueous FGQDs containing 50 μM of the different metal ions in daylight (top) and under UV light (bottom, λex = 360 nm)). (b) Fluorescence spectra of the F1.0-GQDs with different concentrations of Fe3+. (c) Dependence of (F0 – F)/F0 on Fe3+ concentration.

Table 1

Comparison of Fe3+ Sensing Properties of the FGQDs with Those of Recently Reported GQDs

materials	technique in detail	detection limit (μM)	processing time (h)	ref
FGQDs	PLA process using PPy precursors	0.5	<1 h	this work
GQDs	hydrothermal/modified hummer’s method	1.1	>24 h	(42)
N-doped GQDs	ammonia through hydrothermal method	1	> 24 h	(43)
GQDs	chemical oxidation	60	>26 h	(44)
N-doped GQDs	microwave synthesis	100	>27 h	(45)
GQDs	electrochemical synthesis	7.22	>72 h	(46)
Graphitic GQDs	electrochemical synthesis	2	>96 h	(47)
N-doped GQDs	hydrothermal method	0.5	>12 h	(48)
N-doped/amino-functional GQDs	chemical oxidation	0.5	>24 h	(21)

Conclusions

In summary, we successfully demonstrated that surface-passivated and functionalized GQDs with amine N can be achieved via the PLA process using PPy as both the precursor (amine N) and surfactant agent simultaneously. The FGQDs showed smaller particle sizes and narrower size distributions than those of the GQDs owing to the effect of surface passivation produced by a surfactant (PPy). In addition, the XPS results revealed that the FGQDs were functionalized by PPy, which led to the incorporation of the amine N. Owing to the synergistic effect of surface passivation and functionalization, the FGQDs demonstrated highly efficient and sensitive detection ability toward Fe3+ ions with a low LOD of 500 nM. We also proposed a possible mechanism for the formation of the FGQDs based on the plasma plume, cavitation bubble model, and polarity difference between the GQDs and PPy. We believe that our strategy is a one-step method for surface passivation and functionalization of GQDs by the PLA process, allowing successful practical applications, particularly in the field of heavy-metal-ion sensing.

Experimental Section

Preparation of FGQDs and N-Doped GQDs

Graphite flakes were purchased from HQ Graphene (Groningen, The Netherlands), while polypyrrole (PPy) and high-purity ethanol (>99.99%) were purchased from Sigma-Aldrich (St. Louis, Missouri). FGQDs were synthesized by the facile PLA process from graphite flakes in high-purity ethanol with PPy. Typically, 500 mg of graphite flakes were dispersed in 200 mL of high-purity ethanol and PPy with different concentrations (0, 0.5, 1.0, and 1.5 mol %). The pulsed laser ablation was injected into the graphite solution for 30 min at room temperature (about 25–28 °C) in air using a Q-Switched Nd:YAG laser system. The graphite solution was injected by a horizontal pulsed laser beam (355 nm wavelength and third harmonic) at a repetition rate of 10 Hz. The pulse laser width was 10 nm, and the ablation power was 1 J. After completion of the PLA process, the FGQD suspension was dried overnight at 80 °C under vacuum condition. N-doped GQDs were also prepared by the PLA process. Typically, 500 mg of graphite flakes and 1.5 mol % diethylenetriamine (DETA) were dispersed in 200 mL of high-purity ethanol. Thereafter, the same pulsed laser experimental conditions as those set up for the synthesis of the FGQDs were employed.

Characterization

High-resolution transmission electron microscopy (HR-TEM) images of the FGQD samples were captured using a JEM-2100F transmission electron microscope equipped with a field emission gun (200 kV; JEOL). XPS profiles were recorded for both the samples using a VG ESCALAB 220i-XL system (Thermo Fisher Scientific, Waltham). XPS and high-resolution scans were performed at pass energies of 100 and 20 eV, respectively, and at an X-ray beam size of approximately 100 μm. FGQD samples for XPS measurement were prepared on a silicon substrate by the spin-coating method with the rotation speed adjusted to 2000 rpm. The samples were dried overnight in a vacuum oven at 80 °C prior to the measurements. Room-temperature PL spectra were recorded using a PL spectrophotometer (FluoroMax Plus fitted with 150 W xenon arc lamp, HORIBA, Kyoto, Japan) in the wavelength range of 300–800 nm. The PL emission spectra were recorded at excitation wavelengths of 400, 450, and 500 nm. The above experimental details are as per our previously published work.[31]

Fe3+ Detection

All measurements were prepared for FGQD solution with a concentration of 0.05 mg/ml in phosphate-buffered saline (PBS) at pH = 7.4. All metal cations (Ni2+, Fe3+, K+, Al3+, Zn2+, Mn2+, Ag+, Cd2+, Ca2+, Mg2+) dissolved in deionized water. These solutions were mixed with 1 mL of GQD solution for 5 min, and then the optical properties of the samples were investigated by photoluminescence (PL) under 360 nm. The detection sensitivity for Fe3+ ion was assessed by monitoring the PL intensity of FGQD solutions containing different concentrations of Fe3+.