# Fluorescence Turns on-off-on Sensing of Ferric Ion and L-Ascorbic Acid by Carbon Quantum Dots

Authors: Ravin K. Msto; Hazha Omar Othman; Baraa R. Al-Hashimi; Diyar Salahuddin Ali; Dlshad H. Hassan; Aso Q. Hassan; Slim Smaoui
Journal: Journal of Food Quality
 (2023)

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## Abstract

This study used a hydrothermal approach to create a sensitive and focused nanoprobe. Using an “on-off-on” sensing mechanism, the nanoprobe was employed to detect and quantify ferric ions and L-ascorbic acid. Synthesis of the carbon quantum dots was achieved with a single hydrothermal step at 180°C for 24 hours using hot pepper as the starting material. The prepared CQDs showed high fluorescence with a quantum yield of 30% when excited at 350 nm, exhibiting excitation-dependent fluorescence. The emission of the CQDs can be quenched by adding ferric ions, which can be attributed to complex formation leading to nonradiative photoinduced electron transfer (PET). Adding L-ascorbic acid, which can convert ferric ions into ferrous ions, break the complex, and restore the fluorescence of CQD. The linear range and LOD were (10–90)μM and 1 μM for ferric ions, respectively, and L-ascorbic acid’s linear range was (5–100) μM while LOD was 0.1 μM quantification of both substances was accomplished. In addition, orange fruit was used as an actual sample source for ascorbic acid analysis, yielding up to 99% recovery.


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## Body

## 1. Introduction

Fluorescent carbon dots (CDs) include a large family of carbon nanomaterials, such as polymer carbon dots, graphene quantum dots, carbon nanodots, and carbon quantum dots [1]. Because of their distinctive optical characteristics, CDs have drawn significant interest in the pharmacological and biomedical analysis [2]. The compact size, excellent photoluminescence, biocompatibility, photostability, ease of preparation, and low cost of manufacture of CDs make them a promising technology [3]. Because of their unique properties, CDs are suitable for a wide range of applications, including bio(chemical) sensing [1], photocatalysis [4], thermosensing [5], light-emitting diodes [6], and cancer therapy [7].Numerous carbon dot fabrication techniques based on both bottom-up and top-down tactics have been documented in the literature [8–10]. Bottom-up techniques involve creating carbon dots from tiny molecular or atomic precursors under high pressure and temperature [11]. Top-down techniques, on the other hand, refer to fragmenting large carbon sources into little CDs [12]. Examples of both methods are hydrothermal [13–15], solvothermal [16], laser ablation [17], chemical oxidation [18], electrochemical oxidation [19], and arc-discharge [2], which is used commonly in carbon dot preparation.Single-step hydrothermal method is extensively utilized in the synthesis of carbon dots. Nowadays, this method proved to be ecofriendly because it can be categorized under green chemistry, a simple, affordable, and gentle chemical approach for fabricating carbon dots. Generally, the average sizes and shapes of the CQDs are controlled by using specific precursor concentrations, processing temperatures, and reaction times [20–22].L-ascorbic acid, referred to as vitamin C, is a necessary water-soluble vitamin found in foods like citrus fruits [23]. It is important for the human body’s metabolism, growth, tissue repair, biosynthesis, and several physiological activities [24]. Through its abilities as an antioxidant that aids in scavenging free radicals and reactive oxygen species, which can have detrimental effects on human health, it has also discovered a variety of pharmaceutical applications as a supplement for the prevention or treatment of conditions like scurvy, the common cold, and cardiovascular diseases [25]. Therefore, it is crucial to determine the amount of L-ascorbic acid in pharmaceutical products accurately. As a result, numerous quantification techniques have been developed during the past few years, including high-performance liquid chromatography (HPLC) [26], colorimetry [27], capillary electrophoresis (CE) [28], titration [29], electrochemical [30], and fluorescence assays [31]. Although these techniques satisfy the need for sensitivity and precision, their main drawbacks are that they are costly, time-consuming, and require highly qualified personnel. Simple, quick, sensitive, biocompatible, and inexpensive detection techniques are necessary for the analysis of pharmaceutical components (active pharmaceutical ingredients, contaminants, and additives), and these characteristics are intrinsic to the method suggested in this study.In this study, we developed hydrothermally produced highly fluorescent carbon quantum dots (HP-CQDs) from a naturally occurring product, hot pepper. Ferric ions were used to selectively quench the HP-CQDs, and L-ascorbic acid was used to restore them. Analytes and standard recovery percentages can both be determined using fluorometric test techniques. A concept for creating HP-CQDs with a sensing mechanism for both Fe3+ and L-ascorbic acid is shown in Figure 1.Figure 1
Schematic diagram of the hydrothermal preparation of the HP-CQDs, quenching by the presence of ferric ions, which is followed by fluorescence recovery by the presence of L-ascorbic acid.


## 2. Materials and Methods

### 2.1. Chemicals and Reagents

Analytical-grade chemicals and reagents were all employed in this study without further purification. Fresh hot pepper was purchased from a local store, while Fe(NO3)3, FeCl2, MnCl2.2H2O, Hg(NO3)2, CuSO4.5H2O, AlCl3, L-ascorbic acid, caffeine, folic acid, picric acid, and citric acid were purchased from Sigma-Aldrich. Deionized water was used for the dilution operations.


### 2.2. Preparation of HP-CQDs

Hot peppers were chopped and ground to prepare the HP-CQDs in a single hydrothermal process. In a 25 mL Teflon-lined stainless steel autoclave, 1 g of ground hot pepper was added along with 10 ml of deionized water. A dark brown solution was produced when the autoclave was heated to 180°C for 24 hours. The solution was then cooled to room temperature and filtered using 0.22μm filter paper to eliminate any big undissolved particles or contaminants. The filtrate was centrifuged at 12,000 rpm for 15 minutes, followed by 20 hours of dialysis using a 1000 MWCO membrane. The finished product was preserved for further analysis and displayed blue fluorescence emission at 420 nm.


### 2.3. Characterization of HP-CQDs

Agilent Cary Eclipse Fluorescence Spectrophotometer (USA) was used to obtain the fluorescence spectra. At the same time, an Agilent Cary 60 Spectrophotometer (USA) was utilized to record UV-vis absorption spectra using a quartz cuvette with a path length of 1 cm. TEM images were taken using a Philips CM120 Transmission Electron Microscope (Netherlands). FTIR spectra were recorded by Perkin–Elmer Spectrum 2 FTIR Spectrometer (USA) at room temperature. The XRD pattern was obtained using a Philips PW 1730 X-ray Diffractometer (Netherlands).


### 2.4. Detection of the Standard of Fe3+

Acetate buffer was used to keep the pH at three throughout the process, which involved mixing 1 mL of the HP-CQDs with increasing amounts of ferric nitrate solution and incubating the combination for 5 minutes at room temperature. The fluorescence intensity was next assessed in triplicate for each sample of combination.


### 2.5. Detection of Standard L-Ascorbic Acid

In order to bring the pH to 3, 1 mL of the HP-CQDs was combined with 1 mL of a 0.01 M ferric nitrate solution. The mixture was then incubated for 5 minutes at room temperature. The L-ascorbic acid solution was added to the mixture in escalating amounts, and the combination was incubated at room temperature for 15 minutes before the fluorescence intensity was measured.


### 2.6. Assay of L-Ascorbic Acid in Orange Fruit Samples

To test the suggested sensor, orange fruit was used as a sample source for ascorbic acid determination. Ascorbic acid was extracted from the material using a routine procedure that only required minor adjustments [32]. In a nutshell, the orange fruit was properly peeled, weighed, added to 50 ml of 2% oxalic acid, crushed, and centrifuged for 15 minutes at 10,000 rpm. To remove any leftover contaminants, the resultant solution was filtered through a 0.22 μm filter membrane before being stored at 5°C in a refrigerator for future analysis.


## 2.1. Chemicals and Reagents

Analytical-grade chemicals and reagents were all employed in this study without further purification. Fresh hot pepper was purchased from a local store, while Fe(NO3)3, FeCl2, MnCl2.2H2O, Hg(NO3)2, CuSO4.5H2O, AlCl3, L-ascorbic acid, caffeine, folic acid, picric acid, and citric acid were purchased from Sigma-Aldrich. Deionized water was used for the dilution operations.


## 2.2. Preparation of HP-CQDs

Hot peppers were chopped and ground to prepare the HP-CQDs in a single hydrothermal process. In a 25 mL Teflon-lined stainless steel autoclave, 1 g of ground hot pepper was added along with 10 ml of deionized water. A dark brown solution was produced when the autoclave was heated to 180°C for 24 hours. The solution was then cooled to room temperature and filtered using 0.22μm filter paper to eliminate any big undissolved particles or contaminants. The filtrate was centrifuged at 12,000 rpm for 15 minutes, followed by 20 hours of dialysis using a 1000 MWCO membrane. The finished product was preserved for further analysis and displayed blue fluorescence emission at 420 nm.


## 2.3. Characterization of HP-CQDs

Agilent Cary Eclipse Fluorescence Spectrophotometer (USA) was used to obtain the fluorescence spectra. At the same time, an Agilent Cary 60 Spectrophotometer (USA) was utilized to record UV-vis absorption spectra using a quartz cuvette with a path length of 1 cm. TEM images were taken using a Philips CM120 Transmission Electron Microscope (Netherlands). FTIR spectra were recorded by Perkin–Elmer Spectrum 2 FTIR Spectrometer (USA) at room temperature. The XRD pattern was obtained using a Philips PW 1730 X-ray Diffractometer (Netherlands).


## 2.4. Detection of the Standard of Fe3+

Acetate buffer was used to keep the pH at three throughout the process, which involved mixing 1 mL of the HP-CQDs with increasing amounts of ferric nitrate solution and incubating the combination for 5 minutes at room temperature. The fluorescence intensity was next assessed in triplicate for each sample of combination.


## 2.5. Detection of Standard L-Ascorbic Acid

In order to bring the pH to 3, 1 mL of the HP-CQDs was combined with 1 mL of a 0.01 M ferric nitrate solution. The mixture was then incubated for 5 minutes at room temperature. The L-ascorbic acid solution was added to the mixture in escalating amounts, and the combination was incubated at room temperature for 15 minutes before the fluorescence intensity was measured.


## 2.6. Assay of L-Ascorbic Acid in Orange Fruit Samples

To test the suggested sensor, orange fruit was used as a sample source for ascorbic acid determination. Ascorbic acid was extracted from the material using a routine procedure that only required minor adjustments [32]. In a nutshell, the orange fruit was properly peeled, weighed, added to 50 ml of 2% oxalic acid, crushed, and centrifuged for 15 minutes at 10,000 rpm. To remove any leftover contaminants, the resultant solution was filtered through a 0.22 μm filter membrane before being stored at 5°C in a refrigerator for future analysis.


## 3. Results and Discussion

### 3.1. Characterization

The TEM images (Figure2(a)) were utilized to study the morphology of the HP-CQDs, which demonstrated that they have a diameter of around 6 nm. Moreover, the HP-CQDs were also characterized by HR-TEM, obtaining a lattice d-spacing of 0.34 nm (Figure 2(b)) which confirms that the obtained HP-CQDs are of crystalline graphitic nature [33].Figure 2
(a) TEM image and size distribution inset, (b) HR-TEM image and lattice spacing, (c) X-ray diffraction spectrum of the HP-CQDs, and (d) EDX analysis.
(a)(b)(c)(d)The results of X-ray diffraction (XRD) shown in (Figure2(c)) demonstrated a broad peak around 2Ɵ = 23.9°, which can be attributed to the graphite structure of the HP-CQDs [34]. The elemental analysis of the synthesized HP-CQDs shown in Figure 2(d) gives the current weight of each element present in HP-CQDs.Both UV-vis and fluorescence spectroscopy were used to examine the optical characteristics of the HP-CQDs. The UV-vis spectra (Figure3(a)) showed a tiny peak at about 260 nm that results from the electrons’ π = π transition because C=C bonds are present [35]. Furthermore, another broad peak is observed at 345 nm that can be related to n to π∗, which originates from C=O or C–OH present on the surface of the HP-CQDs [36]. According to the fluorescence spectra (Figure 3(b)), HP-CQDs exhibit excitation-dependent emission fluorescence, and their highest emission was seen at 420 nm when excited at 350 nm (Figure 3(c)). L-ascorbic acid can be added to HP-CQDs to reverse the quenching of the fluorescence intensity brought on by the addition of ferric ions; however, the resulting complex of HP-CQDs: Fe3+-L-ascorbic acid has a bathochromic effect on the emission spectra, shifting it to a longer wavelength [37] (Figure 3(d)).Figure 3
(a) UV-vis absorbance spectrum of HP-CQDs in the solution. (b) Absorbance and emission spectrum of HP-CQDs; the excited wavelength was 350 nm. (c) Fluorescence spectrum at different excitation wavelengths. (d) Fluorescence spectra of HP-CQDs (alone), HP-CQDs: Fe3+, and HP-CQDs: Fe3+-L-ascorbic acid.
(a)(b)(c)(d)The functional groups of the HP-CQDs were studied using FTIR spectroscopy (Figure4). Two peaks are observed at 3650 and 3550 cm−1 which can be attributed to the presence of NH2 groups on the surface of the carbon quantum dots. Another characteristic peak is observed at 2120 cm−1, which originates from either C≡C or N≡C bonds, while the bond at 1610 cm−1 can be designated to the C=C group.Figure 4
FTIR spectrum of the HP-CQDs.


### 3.2. Stability of the HP-CQDs

Under a variety of circumstances, including variable pH levels, ionic strengths, radiation levels, and temperatures, the stability of the HP-CQDs fluorescence intensity were investigated. Over the pH range of 1 to 10, the fluorescence intensity did not change much; however, a drop was seen at a strongly alkaline pH of 12 (Figure5(a)), which can be caused by protonation and deprotonation of the surface functional groups [38]. Furthermore, when the carbon dots were dissolved in NaCl solution with an ionic strength of up to 2 M, no discernible changes in the fluorescence intensity of the carbon dots were seen (Figure 5(a)). In addition, a low reduction in fluorescence intensity was seen throughout a wide temperature range of 20 to 145°C, confirming the carbon dots’ thermal resilience (Figure 5 B). The HP-CQDs were then exposed to radiation using a broad-spectrum xenon light (100 mW/cm2), which demonstrated excellent photostability of the carbon dots even after 30 hours of radiation exposure (Figure 5(b)). The HP-CQDs’ stability under extreme pH, temperature, radiation, and ionic strength increases the adaptability and reliability of the suggested analytical approach.Figure 5
(a) Effects of pH and ionic strength on the intensity of HP-CQDs. (b) Effects of temperature and radiation on the intensity of HP-CQDs.
(a)(b)


### 3.3. Determination of Fe3+ and L-Ascorbic Acid

Fe3+ ions were incrementally added to the HP-CQDs solution, and this led to a reduction in the fluorescence emission intensity (Figure 6(a)), showing high linearity in the range of 10 to 90 μM with the R2 value being 0.9949 (Figure 6(b)) and the limit of detection being 1 μM. When L-ascorbic acid was added to the HP-CQDs: Fe3+ complex, the fluorescence emission intensity of the HP-CQDs increased (Figure 6(c)), showing high linearity in the range of 5 to 100 μM with the R2 value being 0.9908 (Figure 6(d)) and the limit of detection being 0.1 μM.Figure 6
(a) Fluorescence spectra of serial solutions of HP-CQDs: Fe3+, the fluorescence spectra were recorded when excited at 350 nm. (b) Calibration graph of HP-CQDs: Fe3+ solutions. (c) Fluorescence spectra of serial solutions of HP-CQDs: Fe3+-L-ascorbic acid, and the fluorescence spectra were recorded when excited at 350 nm. (d) Calibration graph of HP-CQDs: Fe3+-L-ascorbic acid solutions.
(a)(b)(c)(d)


### 3.4. Mechanism of “On-Off-On” Sensing by Fe+3 and L-Ascorbic Acid

To understand the theory behind the off-on phenomenon, it is imperative to study the UV-vis spectra of the HP-CQDs alone and both the HP-CQDs: Fe3+ and HP-CQDs: Fe3+: AA (Figure 7). Because of the formation of a complex between the HP-CQDs and Fe3+, which results in the ferric ions serving as an electron acceptor and causing nonradiative photoinduced electron transfer (PET), the addition of the Fe3+ to the HP-CQDs solution caused considerable alterations in the UV-vis spectra [39]. In addition, the addition of L-ascorbic acid caused the emission fluorescence intensity to recover due to the redox properties of L-ascorbic acid and the conversion of Fe3+ to Fe2+, which cannot form a complex with the HP-CQDs, consequently restoring the fluorescence intensity.Figure 7
UV-vis spectra and inset image describe the mechanism of on-off-on of HP-CQD for sensing of Fe3+ and L-ascorbic acid.


### 3.5. Determination of Ascorbic Acid via Restoring the Fluorescence

Ascorbic acid is simple to detect selectivity and sensitivity by observing the restored fluorescence of HP-CQDs. With increasing AA concentration, the fluorescence restoration of the HP-CQDs: Fe3+ system rises linearly. The relative fluorescence intensity (F/F0) versus ascorbic acid concentration is presented in Figure 8, where F0 and F are the fluorescence intensities of the HP-CQDs–Fe3+ system in the absence and presence of ascorbic acid, respectively.Figure 8
Selectivity of HP-CQDs for ascorbic acid.There was a 0.980 linear correlation coefficient found. The limit of detection for ascorbic acid using this technique is predicted to be 0.1 M for the proposed test. Tables1 and 2 provide a comparison of this sensor’s performance with those of other pertinent sensors.Table 1
Comparison of different carbon dots sensors for the determination of ascorbic acid.
PrecursorMethodSample typeLODLinear rangeReferenceDiethylenetriamine-pentaacetic acidCalcinationWastewater0.15μM2.5–400μM[40]Citric acid and diethylenetriamineSolvothermal—0.03 mmol/L0.03–0.1 mmol/L[31]Citric acid and ethylenediamineHydrothermalJujube fruit3.11μM5–350μM[41]Citric acid and ethylenediamineHydrothermalKiwi fruit3.17μM5–70μM[42]Citric acid and thioureaHydrothermalFruits4.69μM10–200μM[43]Ascorbic acidMicrowave—0.05μM0.2–284μM[44]Hot pepperHydrothermalOrange fruit0.1μM5–100μMThis studyTable 2
Comparison of different carbon dots sensors for the determination of Fe+3.
PrecursorMethodSample typeLOD (μM)Linear rangeReferenceCitric acid and ureaSolvothermalTap water500.1–0.9μM[16]GarlicSolvothermalTap water0.20–500μM[45]Hydroxymethyl aminomethane and glycerinumSolvothermalLake water500.05–3.1 mM[46]Phenylalanine and citric acidHydrothermalTap water0.725.0–500.0μM[47]L - glutamic acid and ethylenediamineMicrowaveTap water3.88–80μM[48]Hot pepperHydrothermal—110–90μMThis study


### 3.6. Selectivity of HP-CQDs and HP-CQDs: Fe3+

The interferences from ions other than Fe3+, such as Mn2+, Al3+, Fe+2, Hg2+,, and Cu2+, were studied by mixing them with the HP-CQDs solution (Figure 9). This demonstrated that the addition of these metals had no effect on the fluorescence intensity and that the HP-CQDs were only selectively quenched by Fe3+ ions. Figure 9 shows the selectivity experiment for L-ascorbic acid detection employing a collection of biomolecules comprising urea, glucose, cysteine, glycine, and citric acid. It can be shown that only L-ascorbic acid can significantly increase PL intensity recovery, proving that the fluorescent probe that was built was extremely selective for L-ascorbic acid.Figure 9
Selectivity of HP-CQDs and HP-CQDs: Fe3+.


### 3.7. Real Sample Detection in Orange Fruit

This sensor was employed to detect ascorbic acid in orange fruit to indicate the reliability of the HP-CQDs: Fe3+ system in ascorbic acid detection. The detection results are displayed in Table 3, in which the spiked recoveries range from 96 to 99%.Table 3
Determination of ascorbic acid in the real sample.
SampleAdded (μM)Found (μM)Error (%)Recovery (%)Orange fruit0.1250.1213.296.80.3100.2983.896.00.5200.5150.9599.00.7100.6853.596.0


## 3.1. Characterization

The TEM images (Figure2(a)) were utilized to study the morphology of the HP-CQDs, which demonstrated that they have a diameter of around 6 nm. Moreover, the HP-CQDs were also characterized by HR-TEM, obtaining a lattice d-spacing of 0.34 nm (Figure 2(b)) which confirms that the obtained HP-CQDs are of crystalline graphitic nature [33].Figure 2
(a) TEM image and size distribution inset, (b) HR-TEM image and lattice spacing, (c) X-ray diffraction spectrum of the HP-CQDs, and (d) EDX analysis.
(a)(b)(c)(d)The results of X-ray diffraction (XRD) shown in (Figure2(c)) demonstrated a broad peak around 2Ɵ = 23.9°, which can be attributed to the graphite structure of the HP-CQDs [34]. The elemental analysis of the synthesized HP-CQDs shown in Figure 2(d) gives the current weight of each element present in HP-CQDs.Both UV-vis and fluorescence spectroscopy were used to examine the optical characteristics of the HP-CQDs. The UV-vis spectra (Figure3(a)) showed a tiny peak at about 260 nm that results from the electrons’ π = π transition because C=C bonds are present [35]. Furthermore, another broad peak is observed at 345 nm that can be related to n to π∗, which originates from C=O or C–OH present on the surface of the HP-CQDs [36]. According to the fluorescence spectra (Figure 3(b)), HP-CQDs exhibit excitation-dependent emission fluorescence, and their highest emission was seen at 420 nm when excited at 350 nm (Figure 3(c)). L-ascorbic acid can be added to HP-CQDs to reverse the quenching of the fluorescence intensity brought on by the addition of ferric ions; however, the resulting complex of HP-CQDs: Fe3+-L-ascorbic acid has a bathochromic effect on the emission spectra, shifting it to a longer wavelength [37] (Figure 3(d)).Figure 3
(a) UV-vis absorbance spectrum of HP-CQDs in the solution. (b) Absorbance and emission spectrum of HP-CQDs; the excited wavelength was 350 nm. (c) Fluorescence spectrum at different excitation wavelengths. (d) Fluorescence spectra of HP-CQDs (alone), HP-CQDs: Fe3+, and HP-CQDs: Fe3+-L-ascorbic acid.
(a)(b)(c)(d)The functional groups of the HP-CQDs were studied using FTIR spectroscopy (Figure4). Two peaks are observed at 3650 and 3550 cm−1 which can be attributed to the presence of NH2 groups on the surface of the carbon quantum dots. Another characteristic peak is observed at 2120 cm−1, which originates from either C≡C or N≡C bonds, while the bond at 1610 cm−1 can be designated to the C=C group.Figure 4
FTIR spectrum of the HP-CQDs.


## 3.2. Stability of the HP-CQDs

Under a variety of circumstances, including variable pH levels, ionic strengths, radiation levels, and temperatures, the stability of the HP-CQDs fluorescence intensity were investigated. Over the pH range of 1 to 10, the fluorescence intensity did not change much; however, a drop was seen at a strongly alkaline pH of 12 (Figure5(a)), which can be caused by protonation and deprotonation of the surface functional groups [38]. Furthermore, when the carbon dots were dissolved in NaCl solution with an ionic strength of up to 2 M, no discernible changes in the fluorescence intensity of the carbon dots were seen (Figure 5(a)). In addition, a low reduction in fluorescence intensity was seen throughout a wide temperature range of 20 to 145°C, confirming the carbon dots’ thermal resilience (Figure 5 B). The HP-CQDs were then exposed to radiation using a broad-spectrum xenon light (100 mW/cm2), which demonstrated excellent photostability of the carbon dots even after 30 hours of radiation exposure (Figure 5(b)). The HP-CQDs’ stability under extreme pH, temperature, radiation, and ionic strength increases the adaptability and reliability of the suggested analytical approach.Figure 5
(a) Effects of pH and ionic strength on the intensity of HP-CQDs. (b) Effects of temperature and radiation on the intensity of HP-CQDs.
(a)(b)


## 3.3. Determination of Fe3+ and L-Ascorbic Acid

Fe3+ ions were incrementally added to the HP-CQDs solution, and this led to a reduction in the fluorescence emission intensity (Figure 6(a)), showing high linearity in the range of 10 to 90 μM with the R2 value being 0.9949 (Figure 6(b)) and the limit of detection being 1 μM. When L-ascorbic acid was added to the HP-CQDs: Fe3+ complex, the fluorescence emission intensity of the HP-CQDs increased (Figure 6(c)), showing high linearity in the range of 5 to 100 μM with the R2 value being 0.9908 (Figure 6(d)) and the limit of detection being 0.1 μM.Figure 6
(a) Fluorescence spectra of serial solutions of HP-CQDs: Fe3+, the fluorescence spectra were recorded when excited at 350 nm. (b) Calibration graph of HP-CQDs: Fe3+ solutions. (c) Fluorescence spectra of serial solutions of HP-CQDs: Fe3+-L-ascorbic acid, and the fluorescence spectra were recorded when excited at 350 nm. (d) Calibration graph of HP-CQDs: Fe3+-L-ascorbic acid solutions.
(a)(b)(c)(d)


## 3.4. Mechanism of “On-Off-On” Sensing by Fe+3 and L-Ascorbic Acid

To understand the theory behind the off-on phenomenon, it is imperative to study the UV-vis spectra of the HP-CQDs alone and both the HP-CQDs: Fe3+ and HP-CQDs: Fe3+: AA (Figure 7). Because of the formation of a complex between the HP-CQDs and Fe3+, which results in the ferric ions serving as an electron acceptor and causing nonradiative photoinduced electron transfer (PET), the addition of the Fe3+ to the HP-CQDs solution caused considerable alterations in the UV-vis spectra [39]. In addition, the addition of L-ascorbic acid caused the emission fluorescence intensity to recover due to the redox properties of L-ascorbic acid and the conversion of Fe3+ to Fe2+, which cannot form a complex with the HP-CQDs, consequently restoring the fluorescence intensity.Figure 7
UV-vis spectra and inset image describe the mechanism of on-off-on of HP-CQD for sensing of Fe3+ and L-ascorbic acid.


## 3.5. Determination of Ascorbic Acid via Restoring the Fluorescence

Ascorbic acid is simple to detect selectivity and sensitivity by observing the restored fluorescence of HP-CQDs. With increasing AA concentration, the fluorescence restoration of the HP-CQDs: Fe3+ system rises linearly. The relative fluorescence intensity (F/F0) versus ascorbic acid concentration is presented in Figure 8, where F0 and F are the fluorescence intensities of the HP-CQDs–Fe3+ system in the absence and presence of ascorbic acid, respectively.Figure 8
Selectivity of HP-CQDs for ascorbic acid.There was a 0.980 linear correlation coefficient found. The limit of detection for ascorbic acid using this technique is predicted to be 0.1 M for the proposed test. Tables1 and 2 provide a comparison of this sensor’s performance with those of other pertinent sensors.Table 1
Comparison of different carbon dots sensors for the determination of ascorbic acid.
PrecursorMethodSample typeLODLinear rangeReferenceDiethylenetriamine-pentaacetic acidCalcinationWastewater0.15μM2.5–400μM[40]Citric acid and diethylenetriamineSolvothermal—0.03 mmol/L0.03–0.1 mmol/L[31]Citric acid and ethylenediamineHydrothermalJujube fruit3.11μM5–350μM[41]Citric acid and ethylenediamineHydrothermalKiwi fruit3.17μM5–70μM[42]Citric acid and thioureaHydrothermalFruits4.69μM10–200μM[43]Ascorbic acidMicrowave—0.05μM0.2–284μM[44]Hot pepperHydrothermalOrange fruit0.1μM5–100μMThis studyTable 2
Comparison of different carbon dots sensors for the determination of Fe+3.
PrecursorMethodSample typeLOD (μM)Linear rangeReferenceCitric acid and ureaSolvothermalTap water500.1–0.9μM[16]GarlicSolvothermalTap water0.20–500μM[45]Hydroxymethyl aminomethane and glycerinumSolvothermalLake water500.05–3.1 mM[46]Phenylalanine and citric acidHydrothermalTap water0.725.0–500.0μM[47]L - glutamic acid and ethylenediamineMicrowaveTap water3.88–80μM[48]Hot pepperHydrothermal—110–90μMThis study


## 3.6. Selectivity of HP-CQDs and HP-CQDs: Fe3+

The interferences from ions other than Fe3+, such as Mn2+, Al3+, Fe+2, Hg2+,, and Cu2+, were studied by mixing them with the HP-CQDs solution (Figure 9). This demonstrated that the addition of these metals had no effect on the fluorescence intensity and that the HP-CQDs were only selectively quenched by Fe3+ ions. Figure 9 shows the selectivity experiment for L-ascorbic acid detection employing a collection of biomolecules comprising urea, glucose, cysteine, glycine, and citric acid. It can be shown that only L-ascorbic acid can significantly increase PL intensity recovery, proving that the fluorescent probe that was built was extremely selective for L-ascorbic acid.Figure 9
Selectivity of HP-CQDs and HP-CQDs: Fe3+.


## 3.7. Real Sample Detection in Orange Fruit

This sensor was employed to detect ascorbic acid in orange fruit to indicate the reliability of the HP-CQDs: Fe3+ system in ascorbic acid detection. The detection results are displayed in Table 3, in which the spiked recoveries range from 96 to 99%.Table 3
Determination of ascorbic acid in the real sample.
SampleAdded (μM)Found (μM)Error (%)Recovery (%)Orange fruit0.1250.1213.296.80.3100.2983.896.00.5200.5150.9599.00.7100.6853.596.0


## 4. Conclusion

In conclusion, utilizing hot pepper as a precursor, extremely fluorescent carbon quantum dots were produced in a single step via green hydrothermal synthesis. The nonradiative photoinduced electron transfer (PET) of the HP-CQDs by the addition of ferric ions, followed by the recovery of fluorescence by the addition of L-ascorbic acid due to the conversion of Fe+3 to Fe+2, is the proposed mechanism for the “off-on” sensing platform. Various extreme circumstances of pH, temperature, radiation, and ionic strength, which had a minor influence on the fluorescence intensity, were used to establish the stability of the carbon dots. Low detection limits of 1 M for Fe+3 and 0.1 M for L-ascorbic acid were achieved, while the measurement of ferric ions and L-ascorbic acid was carried out with good accuracy, precision, and recovery percentage. Finally, a real sample analysis conducted on orange fruit showed a high recovery level of 96–99%.



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