# Boron-Doped Graphene Quantum Dots (BGQDs) from Spent Coffee Ground for Glucose Sensor

Authors: Filimon Hadish; Meng-Hsuan Chiang; Yi-Fang Hsieh; Shao-Yu Wu; Shyankay Jou
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

We have prepared graphene quantum dots (GQDs) and boron-doped GQDs (BGQDs) utilizing spent coffee grounds (SCGs) via a simple one-step hydrothermal process for glucose sensor application. FTIR and XPS characterizations reveal that the boron atoms have been successfully doped into graphene structures. BGQDs on glassy carbon electrode (GCE) in PBS were found to be two times as active compared to GCE/GQDs electrodes. The significant difference in electrochemical activity shown by BGQDs is evidence that boron from boric acid was doped into the graphene dominion. The generation of boronic acid groups on the boron-doped graphene quantum dots (BGQDs) surfaces facilitates the application of GQDs as a new photoluminescence (PL) probe for label-free glucose sensing. The photoluminescence of developed GQDs showed a linear response to glucose over a concentration range of 5–45 mM with a limit of detection of 12.45 mM whereas BGQDs biosensor was found to exhibit a higher sensitivity over the same concentration range with limit of detection of 3.23 mM towards glucose sensing. These results demonstrate that the synthesized BGQD has a promising potential in electrochemical activity and efficient to the PL enhancement mechanism determination of glucose.


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## Body

## 1. Introduction

Diabetes, a group of metabolic diseases manifested by the increase of sugar in blood levels resulting from defects in insulin secretion, insulin action, or both, has been reported to cause dysfunction of the nerve, blood vessels, kidney failure, and loss of eyesight [1]. The uprising in the number of diabetic patients has been a concerning issue for most developed countries and has stimulated the advancement of glucose biosensors [2]. Paying attention to the accurate and rapid detection of glucose, enzymatic glucose oxidase (GOx) electrode sensors were first developed in 1962 [3]. Since then, a wide variety of electrode materials have been reported for glucose sensors, including metal nanoparticles [4], carbon nanotubes [5], and graphene-based nanomaterials [6, 7]. In our previous work, we have reported functionalized CVD-grown single-layer graphene for glucose sensing applications [8]. However, pristine graphene, with its exceptional properties, has limitations in its practical application as an electrode [9]. For example, pristine graphene has a high sheet resistance and low work function, which makes it unsuitable as an electrode. Moreover, graphene is a zero-bandgap semiconductor, and emission of light from its flat layer is almost impossible [10, 11]. As a result, the absence of a band gap in graphene has restricted its electrical and optical characteristics, particularly its emissive properties [12]. To overcome this downside, cutting graphene into nanoscale pieces is a widely employed band gap opening method [13]. This effect is commonly referred to as the quantum confinement effect (QCE) and makes it possible to tune the optical spectra (absorption and photoluminescence, PL) of graphene simply by changing its size and shape while keeping its composition constant. Additionally, the degree of QCE may also vary depending on the shape of the nanoparticle [14].In recent years, graphene quantum dots (GQDs) have drawn incredible research interest owing to their distinctive electronic and optical properties [13]. Further, GQDs exhibit strong and stable PL and low cytotoxicity [15, 16], and their physical and chemical properties can be manipulated through doping, creating defects, and attachment of chemical functionalities [17]. The two main approaches that have been utilized to fabricate GQDs are top-down and bottom-up methods [18]. The top-down synthesis of GQDs is a prominent technique, particularly when graphene, graphene oxide (GO), reduced graphene oxide, carbon nanotubes (CNTs), and graphite are employed as starting materials [13]. In addition to its abundant of starting material and simple operation, top-down method prepared GQDs are rich of functional group at their edge that assist in the functionalization or modification of GQDs surface [19]. Bak et al. [20] assessed various starting materials and synthesis strategies for the preparation of GQD. For instance, hydrothermal, microwave, ultrasonication, and radical synthesis methods can utilize graphene and graphene oxide as preliminary materials. On the other hand, acid-free materials and Hummer’s methods employed graphite, and yet, the unzipping of CNTs and carbon fibers (CFs) can be carried out in the synthesis of GQD [20]. Hydrothermal approaches have been reported to be efficient for the cutting of graphene sheets into surface-functionalized GQDs (ca. 9.6 nm average diameter) [21]. The synthesis method may have incorporated the complete breakup of mixed epoxy chains composed of fewer epoxy groups and more carbonyl groups. The functionalized GQDs were found to exhibit bright blue PL [21]. The bottom-up methods are efficient routes to produce GQDs by utilizing small molecules. Synthesis approaches such as hydrothermal, microwave, and combustion methods, pyrolysis in concentrated acid, carbonization in a microreactor, and enhanced hydrothermal (microwave-hydrothermal and plasma-hydrothermal) methods have been reported [18, 19]. Moreover, starting materials such as glucose [22], plant leaf [23], and recently broccoli [24] have been utilized in the synthesis of GQDs. However, few researchers synthesized GQDs from SCG, but none of them were articulated for biosensor application [25, 26].Spent coffee grounds (SCGs) are sludge and filtered to dry from the coffee brewing process, often considered as waste material and usually end up in landfilling purpose. It has been reported that value adding research works such as those on domestic ash from biomass combustion, on soil fertility and plant growth [27], on the recovery of energy as well as the production of biochar as an adsorbent [28], and geopolymers [29], have been developed. Additionally, in situ hybrid nanocomposites obtained through carbonization of SCG institute an efficient electrode material for batteries application [30].Here, we prepared GQDs with a simple one-step hydrothermal process of SCG for the glucose sensor application. Similarly, SCG and boric acid mixture as the precursor also hydrothermally treated at 200°C, resulting in the production of boron-doped graphene quantum dots (BGQDs) with superior photoluminescence activity towards glucose in a controlled boron concentration. The PL intensity of GQDs and BGQDS were found to decrease with the increase of glucose concentrations. Furthermore, the band gap of GQDs and BGQDs was investigated from the cyclic voltammetry response, and well-defined peaks were obtained vs. the Ag/AgCl reference electrode.


## 2. Preparation of GQDs and BGQDs

SCG was collected from filter papers coming out of coffee machines and dried at 100°C overnight. The dried coffee waste was sieved with 125μm pore size mesh to obtain uniform fine powder and dispersed in DI water of 1 : 6 weight ratios for 3 hours. The mixture was treated by a one-step hydrothermal method in a Teflon-lined stainless steel autoclave at 200°C for 10 h (see Figure 1) to synthesize GQDs solution [21]. Similar procedures were followed when boric acid (10 wt%) was added as a dopant to prepare BGQDs. Glucose with various concentrations and GOx type VII from Aspergillus niger were added to the GQDs and BGQDs solutions, and the PL was analyzed for glucose sensing. Cyclic voltammetric (CV) experiments were performed in a phosphate buffer solution (PBS) using a three-electrode configuration. GQDs and BGQDs with a modified glassy carbon electrode were used as the working electrode, and a platinum wire and Ag/AgCl were employed as the counter and reference electrodes, respectively.Figure 1
Schematic representation of GQDs and BGQDs synthesis of SCG.


## 3. Results and Discussion

### 3.1. TEM Morphology Analysis

Figures2(a) and 2(b) display the typical HRTEM images of the SCG-derived BGQDs (synthesized hydrothermally of reaction temperature at 200°C), showing relatively narrow size distribution between 3.8 and 6.5 nm, and the obvious crystal lattice presents high crystallinity of BGQDs with the lattice spacing of 0.158 nm.Figure 2
HRTEM images of BGQDs prepared in a single step hydrothermal process.
(a)(b)


### 3.2. Optical Characterization of GQDs and BGQDs

The optical properties of both the GQDs and BGQDs were characterized as shown in Figure3. The SCG-prepared GQDs shown in Figure 3(a) exhibit a single UV-visible absorption peak at 270 nm, attributed to π−π∗ electronic transition and consistent with literature [31]. Whereas, BGQDs depicted in Figure 3(c) exhibited two strong UV-visible absorption peaks around 275 nm red shifted ca. 2 nm, than other reports [32], and the shoulder peak at 326 nm blue shifted ca. 19 nm, respectively, attributed to the π−π∗ transitions of the C=C bond and n−π∗ transitions of the C=O bond [33]. The inset photographs of Figures 3(a) and 3(c) show that both GQDs and BGQDs solutions are pale-yellow, transparent, and clear under daylight, and exhibit pacific blue PL under irradiation by a 365 nm UV light. Moreover, the PL of GQDs and BGQDs at different excitation wavelengths showed a redshift with an increase in excitation energy. At an excitation wavelength of 310 nm, the emission maxima of GQDs and BGQDs were located at 396 and 397 nm, respectively. However, the quenching study was conducted at a higher excitation wavelength of 340 nm to ensure consistency with previous glucose-based GQDs and BGQDs [34].Figure 3
UV-vis absorption of (a) GQDs and (c) BGQDs prepared from SCG (200°C) and excitation-dependent PL behavior of (b) GQDs from 280 to 400 nm and (d) BGQDs from 280 to 430 nm. Inset photographs of GQDs solution in (a) and BGQDs solution in (c) were taken under visible and 365 nm UV light, respectively.
(a)(b)(c)(d)


### 3.3. PL Quenching of GQDs and BGQDs as a Probe Sensor to Glucose

Figures4(a) and 4(b) illustrate PL spectrum of as-prepared GQDs and BGQDs obtained at an excitation wavelength of 370 nm and 340 nm, respectively. The emission wavelength centered at 493 nm and 417 nm, corresponding to GQDs and BGQDs, remains constant while the intensity changes in response to glucose concentrations of 0, 5, 10, 15, 20, 25, 30, 35, 40, and 45 mM. The decrease in PL intensity is associated with the PL quenching of GQDs and BGQDs.Figure 4
The fluorescence quenching spectra of (a) GQDs and (b) BGQDs in the detection 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 mM concentrations of glucose. (c, d) The respective plot of the fluorescence quenching for GQDs atλ = 439 and BGQDs at 417 nm vs. the concentration of glucose.
(a)(b)(c)(d)Glucose was oxidized to gluconic acid and H2O2 in the presence of GOx, as shown in following equation:(1)glucose+O2+H2O⟶GOxgluconicacid+H2O2,Thus, H2O2 quenched GQDs fluorescence, and the effect was proportional to the concentration of glucose, as shown in Figure 4(c). According to the Stern–Volmer equation, F0/F = 1 + KSV [Q], where F0 and F are the PL intensities observed in the absence and presence of quencher, respectively, KSV is the Stern–Volmer quenching constant, and [Q] is the concentration of quencher [31]. The linear regression equation shown in Figure 4(c) for the respective concentration ranges is given by Y = –0.0016 [glucose] (mM) + 0.99244 with a correlation coefficient of 0.926. Thus, the GQDs biosensor synthesized from spent coffee ground exhibited the sensitivity over the wide range of glucose concentration with a limited detection (LOD) of 12.45 mM. However, when the GQDs were doped with only 10% boric acid, the linear regression equation of Y = −0.00772[glucose]/(mM) + 1.00629 with a higher regression coefficient of 0.99532 and a detection limit of 3.23 mM was obtained, as shown in Figure 4(d). Thus, the data revealed that BGQDs for the same concentration range delivered higher sensitivity in response to glucose compared to boron-free GQDs. Moreover, BGQDs were found to be 26% more efficient in the detection of small concentrations of glucose than GQDs.


### 3.4. Raman and FTIR Characterization

Raman spectra (Figure5) further confirm the quality of the as-prepared GQDs and BGQDs. Both GQDs show the D band at 1356 cm−1 related to the presence of sp3 defects (disordered structures of carbon) and the crystalline (G) band at 1583 cm−1 related to the in-plane vibration of sp2 carbon (the graphitic structures). The ratio of the intensities (IG/ID) of these characteristic bands can be used to correlate the structural properties of the graphene. The IG/ID is 1.13 and 2.38 for GQDs and BGQDs, respectively. These results indicate that BGQDs may have fewer defects than GQDs [35, 36].Figure 5
(a) Raman and (b) FTIR spectrum of GQDs and BGQDs.
(a)(b)Fourier transform infrared (FTIR) spectroscopy of GQDs and BGQDs was performed to characterize the functional groups they comprised. The FTIR spectra in Figure5(a) show a broad peak at about 3346 and 3347 cm−1 that is attributed to the O-H band overlapping the N-H stretching vibration [33]. Furthermore, the existence of a peak at 1381 cm−1 corresponding to the –COO symmetric stretching along with the broad O-H stretching showed a typical peak of the oxygen functional groups of graphene [22]. The two sharp and intense peaks located at 2919 and 2850 cm−1 ascribed to the C-H vibrational stretching [37]. The peak at 1612 cm−1 can be related to the C=C aromatic stretching vibration of graphite domains, and the peak at 1458 cm−1 is associated with the stretching of C-N [38]. The bands at 1708 and 1059 cm−1 are assigned to the vibrational absorption bands of C=O and C-O (alkoxy), respectively [24, 37]. Thus, the functional groups indicate the adequate presence of amino and hydroxyl groups on the surfaces of the two GQDs, which results in GQDs with a good hydrophilic nature [35]. In contrast, the amine and carboxyl groups hinder the intensity of the boron-associated band in the case of BGQDs [39]. However, compared to boron-free GQDs, the intensity of the peaks at 1290 cm−1 depicted in Figure 5(b), which are associated with C-B-C asymmetric stretching [40] vibration, is observed in the case of BGQDs, suggesting that the boron was successfully doped into the graphene domain.


### 3.5. XPS Characterization

X-ray photoelectron spectroscopy (XPS) measurement was employed to probe the chemical composition and configuration of the GQDs and BGQDs. Figure6(a) shows the XPS survey scans for the GQDs and BGQDs. The peaks of C1s, N1s, and O1s observed in the survey scan of both the un-doped GQDs and BGQD samples clearly exhibit C1s, N1s, and O1s peaks centered at 285, 533, and 400 eV, respectively. The survey scan of BGQDs failed to reveal B1s peak, probably. SCG excessively contains cellulose, hemicellulose, lignin, and fats as principal components [41] and is rich in N and O elements, whose peaks predominantly appeared in the XPS survey scan. The C to O ratio of GQDs and BGQDs is 2.3 and 2.68, respectively. The increase in C/O indicated that the amount of oxygen is decreasing from GQDs to BGQDs, which is consistent with elemental analysis results.Figure 6
(a) The XPS wide scan of GQDs and BGQDs (b–h) are C1s, N1s, O1s, and B1s of GQDs and BGQDs, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)The high resolution C1s spectrum of GQDs and BGQDs fitted into six and seven peaks respectively shown in Figures6(b) and 6(e) that corresponds to sp2 C=C (284–284.4 eV), sp3 C-C (285 eV), C–N (285.8 eV), C-O (286.6 eV), carbonyl C=O (287.9), O–C=O (289 eV) in the carboxyl group [8] as well as C-B (283.9 eV) [42]. The relative intensities of different constituents C1s structures are calculated from the deconvoluted XPS spectra and are summarized in Table 1. It can be seen that the C/O relatively decreases from GQDs to BGQDs. Figures 6(d) and 6(g) of O1s peaks in the range of 533.2 and 533.7 eV and are assigned to to the carbonyl (C=O) group, while the peaks in the range of 530 and 532 eV are assigned to the quinone-O group for GQDs and BGQDs, respectively [43]. The lower percentage composition of carbonyl groups in BGQDs might be due to the dehydrolysis of neighboring carboxyl groups contributing to enhancing the intensity of quinone-O in BGQDs [43].Table 1
Summary of C1s core level XPS spectra of GQDs and BGQDs.
SamplesC1s functional groupBinding energy (eV)Relative intensity (%)GQDsC=C284.0323.99C-C285.2037.62C-N285.8617.98C-O286.6612.75C=O287.905.31COOH289.032.33BGQDsC-B283.9413.70C=C284.4125.63C-C285.1831.90C-N285.8713.57C-O286.627.80C=O287.884.71COOH288.972.67BC4187.527.70BC3189.1825.93BC2O190.2132.48BCO2191.3733.87The structural feature of the N is the principal component in the XPS wide scan, and its high-resolution spectrum is investigated. As shown in Figures6(c) and 6(f), the N 1s spectra that correspond to GQDs and BGQDs can be deconvoluted into two peaks centered at 399.9 and 401 eV, which are assigned to pyrrolic-N and graphitic-N [44], respectively. The relative amount of pyrrolic-N increases and that of graphitic-N decreases from GQDs to BGQDs samples, which might be due to the boron doping effect. Finally, the high-resolution B1s spectrum of BGQDs can be deconvoluted into four peaks, which are displayed in Figure 6(h). The peak at binding energy of 187.5 and 189.2 eV are assigned to BC4 and BC3 structures, respectively [45]. Further peaks at 190.2 and 191.4 eV are attributed to BC2O and BCO2 structures [39]. The contents of the different B–C bond structures are calculated from the deconvoluted XPS spectra and are summarized in Table 1. It can be seen that BGQDs samples have a large portion of the oxidized B–C bond, such as BC2O and BCO2 structures, together with the graphite-like BC3 structure.


### 3.6. Electrochemical Characterization

Figure7 shows the cyclic voltammetry (CV) response to a glassy carbon electrode (GCE) modified with GQDs and BGQDs in 0.1 M PBS against an Ag/AgCl electrode. The current response obtained with the GCE/BGQDs electrode is 10 times higher than that of the GCE/GQDs electrode. Precise quantitative measurements from the CV response peaks were employed to determine the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of both GQDs and BGQDs. The technique is mainly utilized to compute the band gap energy of the prepared quantum dots.Figure 7
Cyclic voltammetry measurements of GQDs/GCE and BGQDs/GCE in 0.1 M PBS vs. Ag/AgCl.Accordingly, the GCE/GQDs electrode displayed a reduction peak at −0.5 V and an oxidation peak at +0.35 V versus Ag/AgCl in 0.1 M PBS in the potential window of −1.5 to 1.5 V, as shown in the inset of Figure7. The values for LUMO energy (ELUMO) and HOMO energy (EHOMO) calculated based on the relation between Fermi level and NHE [46] are given as follows:(2)EHOMO=−eEOx,Vs.NHE+4.75eV,ELUMO=−eERed,Vs.NHE+4.75eV,ENHE=−eEAgAgCl+0.197eV.As a result,ELUMO and EHOMO estimated at −4.447 eV and −5.297 eV, respectively. Thus, for the GQDs, the band-gap energy determined (EHOMO−ELUMO) is 0.85 eV. However, with the GCE/BGQDs electrode, relatively wider potential oxidation and reduction peaks of −0.7 V and +0.95 V, respectively, were measured. The band gap for BGQDs is found to be 1.65 eV, which is twice that of GQDs. This confirmed that boron as a doping agent increases the electrochemical activity when utilized as a modified electrode. More importantly, boron (BGQDs) plays a vital role in widening the GQDs band gap.


## 3.1. TEM Morphology Analysis

Figures2(a) and 2(b) display the typical HRTEM images of the SCG-derived BGQDs (synthesized hydrothermally of reaction temperature at 200°C), showing relatively narrow size distribution between 3.8 and 6.5 nm, and the obvious crystal lattice presents high crystallinity of BGQDs with the lattice spacing of 0.158 nm.Figure 2
HRTEM images of BGQDs prepared in a single step hydrothermal process.
(a)(b)


## 3.2. Optical Characterization of GQDs and BGQDs

The optical properties of both the GQDs and BGQDs were characterized as shown in Figure3. The SCG-prepared GQDs shown in Figure 3(a) exhibit a single UV-visible absorption peak at 270 nm, attributed to π−π∗ electronic transition and consistent with literature [31]. Whereas, BGQDs depicted in Figure 3(c) exhibited two strong UV-visible absorption peaks around 275 nm red shifted ca. 2 nm, than other reports [32], and the shoulder peak at 326 nm blue shifted ca. 19 nm, respectively, attributed to the π−π∗ transitions of the C=C bond and n−π∗ transitions of the C=O bond [33]. The inset photographs of Figures 3(a) and 3(c) show that both GQDs and BGQDs solutions are pale-yellow, transparent, and clear under daylight, and exhibit pacific blue PL under irradiation by a 365 nm UV light. Moreover, the PL of GQDs and BGQDs at different excitation wavelengths showed a redshift with an increase in excitation energy. At an excitation wavelength of 310 nm, the emission maxima of GQDs and BGQDs were located at 396 and 397 nm, respectively. However, the quenching study was conducted at a higher excitation wavelength of 340 nm to ensure consistency with previous glucose-based GQDs and BGQDs [34].Figure 3
UV-vis absorption of (a) GQDs and (c) BGQDs prepared from SCG (200°C) and excitation-dependent PL behavior of (b) GQDs from 280 to 400 nm and (d) BGQDs from 280 to 430 nm. Inset photographs of GQDs solution in (a) and BGQDs solution in (c) were taken under visible and 365 nm UV light, respectively.
(a)(b)(c)(d)


## 3.3. PL Quenching of GQDs and BGQDs as a Probe Sensor to Glucose

Figures4(a) and 4(b) illustrate PL spectrum of as-prepared GQDs and BGQDs obtained at an excitation wavelength of 370 nm and 340 nm, respectively. The emission wavelength centered at 493 nm and 417 nm, corresponding to GQDs and BGQDs, remains constant while the intensity changes in response to glucose concentrations of 0, 5, 10, 15, 20, 25, 30, 35, 40, and 45 mM. The decrease in PL intensity is associated with the PL quenching of GQDs and BGQDs.Figure 4
The fluorescence quenching spectra of (a) GQDs and (b) BGQDs in the detection 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 mM concentrations of glucose. (c, d) The respective plot of the fluorescence quenching for GQDs atλ = 439 and BGQDs at 417 nm vs. the concentration of glucose.
(a)(b)(c)(d)Glucose was oxidized to gluconic acid and H2O2 in the presence of GOx, as shown in following equation:(1)glucose+O2+H2O⟶GOxgluconicacid+H2O2,Thus, H2O2 quenched GQDs fluorescence, and the effect was proportional to the concentration of glucose, as shown in Figure 4(c). According to the Stern–Volmer equation, F0/F = 1 + KSV [Q], where F0 and F are the PL intensities observed in the absence and presence of quencher, respectively, KSV is the Stern–Volmer quenching constant, and [Q] is the concentration of quencher [31]. The linear regression equation shown in Figure 4(c) for the respective concentration ranges is given by Y = –0.0016 [glucose] (mM) + 0.99244 with a correlation coefficient of 0.926. Thus, the GQDs biosensor synthesized from spent coffee ground exhibited the sensitivity over the wide range of glucose concentration with a limited detection (LOD) of 12.45 mM. However, when the GQDs were doped with only 10% boric acid, the linear regression equation of Y = −0.00772[glucose]/(mM) + 1.00629 with a higher regression coefficient of 0.99532 and a detection limit of 3.23 mM was obtained, as shown in Figure 4(d). Thus, the data revealed that BGQDs for the same concentration range delivered higher sensitivity in response to glucose compared to boron-free GQDs. Moreover, BGQDs were found to be 26% more efficient in the detection of small concentrations of glucose than GQDs.


## 3.4. Raman and FTIR Characterization

Raman spectra (Figure5) further confirm the quality of the as-prepared GQDs and BGQDs. Both GQDs show the D band at 1356 cm−1 related to the presence of sp3 defects (disordered structures of carbon) and the crystalline (G) band at 1583 cm−1 related to the in-plane vibration of sp2 carbon (the graphitic structures). The ratio of the intensities (IG/ID) of these characteristic bands can be used to correlate the structural properties of the graphene. The IG/ID is 1.13 and 2.38 for GQDs and BGQDs, respectively. These results indicate that BGQDs may have fewer defects than GQDs [35, 36].Figure 5
(a) Raman and (b) FTIR spectrum of GQDs and BGQDs.
(a)(b)Fourier transform infrared (FTIR) spectroscopy of GQDs and BGQDs was performed to characterize the functional groups they comprised. The FTIR spectra in Figure5(a) show a broad peak at about 3346 and 3347 cm−1 that is attributed to the O-H band overlapping the N-H stretching vibration [33]. Furthermore, the existence of a peak at 1381 cm−1 corresponding to the –COO symmetric stretching along with the broad O-H stretching showed a typical peak of the oxygen functional groups of graphene [22]. The two sharp and intense peaks located at 2919 and 2850 cm−1 ascribed to the C-H vibrational stretching [37]. The peak at 1612 cm−1 can be related to the C=C aromatic stretching vibration of graphite domains, and the peak at 1458 cm−1 is associated with the stretching of C-N [38]. The bands at 1708 and 1059 cm−1 are assigned to the vibrational absorption bands of C=O and C-O (alkoxy), respectively [24, 37]. Thus, the functional groups indicate the adequate presence of amino and hydroxyl groups on the surfaces of the two GQDs, which results in GQDs with a good hydrophilic nature [35]. In contrast, the amine and carboxyl groups hinder the intensity of the boron-associated band in the case of BGQDs [39]. However, compared to boron-free GQDs, the intensity of the peaks at 1290 cm−1 depicted in Figure 5(b), which are associated with C-B-C asymmetric stretching [40] vibration, is observed in the case of BGQDs, suggesting that the boron was successfully doped into the graphene domain.


## 3.5. XPS Characterization

X-ray photoelectron spectroscopy (XPS) measurement was employed to probe the chemical composition and configuration of the GQDs and BGQDs. Figure6(a) shows the XPS survey scans for the GQDs and BGQDs. The peaks of C1s, N1s, and O1s observed in the survey scan of both the un-doped GQDs and BGQD samples clearly exhibit C1s, N1s, and O1s peaks centered at 285, 533, and 400 eV, respectively. The survey scan of BGQDs failed to reveal B1s peak, probably. SCG excessively contains cellulose, hemicellulose, lignin, and fats as principal components [41] and is rich in N and O elements, whose peaks predominantly appeared in the XPS survey scan. The C to O ratio of GQDs and BGQDs is 2.3 and 2.68, respectively. The increase in C/O indicated that the amount of oxygen is decreasing from GQDs to BGQDs, which is consistent with elemental analysis results.Figure 6
(a) The XPS wide scan of GQDs and BGQDs (b–h) are C1s, N1s, O1s, and B1s of GQDs and BGQDs, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)The high resolution C1s spectrum of GQDs and BGQDs fitted into six and seven peaks respectively shown in Figures6(b) and 6(e) that corresponds to sp2 C=C (284–284.4 eV), sp3 C-C (285 eV), C–N (285.8 eV), C-O (286.6 eV), carbonyl C=O (287.9), O–C=O (289 eV) in the carboxyl group [8] as well as C-B (283.9 eV) [42]. The relative intensities of different constituents C1s structures are calculated from the deconvoluted XPS spectra and are summarized in Table 1. It can be seen that the C/O relatively decreases from GQDs to BGQDs. Figures 6(d) and 6(g) of O1s peaks in the range of 533.2 and 533.7 eV and are assigned to to the carbonyl (C=O) group, while the peaks in the range of 530 and 532 eV are assigned to the quinone-O group for GQDs and BGQDs, respectively [43]. The lower percentage composition of carbonyl groups in BGQDs might be due to the dehydrolysis of neighboring carboxyl groups contributing to enhancing the intensity of quinone-O in BGQDs [43].Table 1
Summary of C1s core level XPS spectra of GQDs and BGQDs.
SamplesC1s functional groupBinding energy (eV)Relative intensity (%)GQDsC=C284.0323.99C-C285.2037.62C-N285.8617.98C-O286.6612.75C=O287.905.31COOH289.032.33BGQDsC-B283.9413.70C=C284.4125.63C-C285.1831.90C-N285.8713.57C-O286.627.80C=O287.884.71COOH288.972.67BC4187.527.70BC3189.1825.93BC2O190.2132.48BCO2191.3733.87The structural feature of the N is the principal component in the XPS wide scan, and its high-resolution spectrum is investigated. As shown in Figures6(c) and 6(f), the N 1s spectra that correspond to GQDs and BGQDs can be deconvoluted into two peaks centered at 399.9 and 401 eV, which are assigned to pyrrolic-N and graphitic-N [44], respectively. The relative amount of pyrrolic-N increases and that of graphitic-N decreases from GQDs to BGQDs samples, which might be due to the boron doping effect. Finally, the high-resolution B1s spectrum of BGQDs can be deconvoluted into four peaks, which are displayed in Figure 6(h). The peak at binding energy of 187.5 and 189.2 eV are assigned to BC4 and BC3 structures, respectively [45]. Further peaks at 190.2 and 191.4 eV are attributed to BC2O and BCO2 structures [39]. The contents of the different B–C bond structures are calculated from the deconvoluted XPS spectra and are summarized in Table 1. It can be seen that BGQDs samples have a large portion of the oxidized B–C bond, such as BC2O and BCO2 structures, together with the graphite-like BC3 structure.


## 3.6. Electrochemical Characterization

Figure7 shows the cyclic voltammetry (CV) response to a glassy carbon electrode (GCE) modified with GQDs and BGQDs in 0.1 M PBS against an Ag/AgCl electrode. The current response obtained with the GCE/BGQDs electrode is 10 times higher than that of the GCE/GQDs electrode. Precise quantitative measurements from the CV response peaks were employed to determine the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of both GQDs and BGQDs. The technique is mainly utilized to compute the band gap energy of the prepared quantum dots.Figure 7
Cyclic voltammetry measurements of GQDs/GCE and BGQDs/GCE in 0.1 M PBS vs. Ag/AgCl.Accordingly, the GCE/GQDs electrode displayed a reduction peak at −0.5 V and an oxidation peak at +0.35 V versus Ag/AgCl in 0.1 M PBS in the potential window of −1.5 to 1.5 V, as shown in the inset of Figure7. The values for LUMO energy (ELUMO) and HOMO energy (EHOMO) calculated based on the relation between Fermi level and NHE [46] are given as follows:(2)EHOMO=−eEOx,Vs.NHE+4.75eV,ELUMO=−eERed,Vs.NHE+4.75eV,ENHE=−eEAgAgCl+0.197eV.As a result,ELUMO and EHOMO estimated at −4.447 eV and −5.297 eV, respectively. Thus, for the GQDs, the band-gap energy determined (EHOMO−ELUMO) is 0.85 eV. However, with the GCE/BGQDs electrode, relatively wider potential oxidation and reduction peaks of −0.7 V and +0.95 V, respectively, were measured. The band gap for BGQDs is found to be 1.65 eV, which is twice that of GQDs. This confirmed that boron as a doping agent increases the electrochemical activity when utilized as a modified electrode. More importantly, boron (BGQDs) plays a vital role in widening the GQDs band gap.


## 4. Conclusion

We have prepared graphene quantum dots (GQDs) and boron-doped BGQDs utilizing spent coffee grounds via a simple one-step hydrothermal process for glucose sensor application. Characterization techniques such as Rama, FTIR, and XPS spectra confirmed that the boron atom from boric acid is successfully doped into the graphene dominion. Further, the photoluminescence of developed GQDs showed a linear response to glucose over a concentration range of 5–45 mM and exhibited a limit of detection of 12.45 mM whereas BGQDs biosensor over the same concentration range showed higher sensitivity with a LOD of 3.23 mM towards glucose sensing. Thus, 26% more efficiency in the detection of small concentrations of glucose is achieved when boron is doped into the GQDs domain. Moreover, the electrochemical response of a BGQD-modified electrode indicated that boron as a doping agent plays a vital role in opening the band gap, which is twice that of GQDs. Thus, the effective utilization of SCG, together with the simplicity of the synthetic strategy and the potential amenability to surface functionalization with boron in the GQDs, can further expand the application of BGQDs in a wide range of sensors and optoelectronics.



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*Source: 4574772-2022-12-17.xml*