Leftover Kiwi Fruit Peel-Derived Carbon Dots as a Highly Selective Fluorescent Sensor for Detection of Ferric Ion †

: Recently, the use of natural products for the synthesis of carbon dots (CDs) has received much attention. Herein, leftover kiwi ( Actinidia Deliciosa ) fruit peels were successfully turned into beneﬁcial ﬂuorescent carbon dots (KN-CDs) via the hydrothermal-carbonization route. KN-CDs 1 and KN-CDs 2 were prepared without and with ammonium hydroxide, respectively. KN-CDs 1 and KN-CDs 2 were systematically characterized by various analytical techniques. Synthesized KN-CDs showed spherical-shaped morphology with narrow size distribution and excellent optical properties with excitation-independent behaviors. The quantum yields of KN-CDs 1 and KN-CDs 2 were calculated as 14 and 19%, respectively. Additionally, the KN-CDs possess excellent prolonging and photostability. Because of the excellent optical properties of KN-CDs, they were utilized as ﬂuorescent sensors. The strong ﬂuorescence of the KN-CDs was selectively quenched by Fe 3+ ion, and quenching behavior showed a linear correlation with the concentrations of Fe 3+ ion. KN-CDs 1 and KN-CDs 2 showed the detection of Fe 3+ ions within the concentration range of 5–25 µ M with the detection limit of 0.95 and 0.85 µ M, respectively. Based on the turn-off sensing by the detection of Fe 3+ ions, KN-CDs would be a promising candidate as a selective and sensitive ﬂuorescent sensor.

The heteroatoms in CDs improve the optical properties; thus, the use of natural products is attractive in the synthesis of CDs by the hydrothermal method [26][27][28]. The properties of CDs mainly depend on their precursors. Naturally, plant sources are composed of many functional groups; thus, by the use of plant parts as a carbon source, it is expected that the prepared CDs will be enriched with heteroatoms. This is believed due to the excellent properties, and further surface passivation is not required [11,29,30]. In addition, by the use of plant parts, the usage of toxic or harsh chemicals can be eliminated. Many reports are available regarding the synthesis of CDs by hydrothermal treatment using plant parts such as flowers, leaves, roots, stems, and seeds [27,31,32]. Therefore, the natural carbon source is still explored for the synthesis of CDs, and CDs with appreciable properties are highly desirable.
Detection of ferric (Fe 3+ ) ions is among the important issues because Fe 3+ ions are essential for human health and the environment; however, excess or dearth of Fe 3+ ions will create serious health and environmental problems [29,31]. Currently, there are various detection methods available for the detection of Fe 3+ ions, such as electrochemistry protocols, as well as various spectrophotometric assays, including inductively coupled plasma atomic emission spectrometry and atomic absorption spectrophotometry. However, these detection methods require sophisticated instrumentation, complicated sample preparation, expensive reagents, and are time consuming. Hence, fluorescence-based sensing methods are preferable because they possess several advantages such as high signal output, easy monitoring, simplicity, rapid response, and cost-effectiveness [29,31]. Thus, developing a simple, reliable fluorescence sensor for the detection of Fe 3+ ions is highly essential.
Here, leftover kiwi fruit peel was used to prepare KN-CDs by the hydrothermalassisted carbonization route. To increase the nitrogen content in KN-CDs, ammonium hydroxide (NH 4 OH) was used. KN-CDs 1 and KN-CDs 2 were synthesized without and with NH 4 OH, respectively. The obtained KN-CDs were thoroughly characterized through various analytical techniques to know their surface morphologies, size, and optical properties. Further, KN-CDs were utilized as metal sensors. To sense the excess metal ions in the human body or to detect the heavy metal ions in the environment, CDs are used [33][34][35]. In general, excess or deficiency of metal ions in an aqueous medium has a potential impact on the environment and human health. Hence, a sensing metal ion in aqueous media is crucial and necessary. In this regard, the prepared KN-CDs were used as potential fluorescent probes for the detection of Fe 3+ ions.

Synthesis of KN-CDs
The KN-CDs were synthesized using the hydrothermal-assisted carbonization route. In detail, leftover kiwi fruit peel was crushed well with deionized water using a commercial mixer grinder. The obtained mucilage (approximately 70 mL) was collected in a 100 mL capacity Teflon-lined stainless-steel autoclave and then heated for 24 h at 200 • C. Successively, the autoclave was cooled to room temperature. The hydrothermally carbonized suspension of kiwi fruit peel was filtered using a cellulose membrane with a pore size of 0.22 µM to remove the larger-sized carbon particles. The resultant brownish-yellow supernatant (KN-CDs 1) was transferred to a glass vial and stored in a refrigerator at 5 • C for further characterization. On the other hand, KN-CDs 2 was prepared by adopting the same procedure as that of KN-CDs 1 with the inclusion of NH 4 OH. Approximately 1.0 mL of NH 4 OH was added to the suspension, and the pH was adjusted to around 8. Scheme 1 shows an illustration of the procedure of KN-CDs synthesis from the leftover kiwi fruit peel.

Results and Discussion
The KN-CDs were prepared using kiwi peel by hydrothermal treatment at 200 • C for a period of 24 h. KN-CDs 1 and KN-CDs 2 were prepared by the absence and presence of NH 4 OH, respectively. The prepared KN-CDs 1 and KN-CDs 2 were characterized by various techniques, such as X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). Figure 1a shows the XRD patterns of the KN-CDs (KN-CDs 1 and KN-CDs 2). The presence of amorphous carbon reflects from the broad peaks centered at~2θ around 22.5 and 42.5 • , which corresponds to (002) and (100) planes, respectively [36,37]. The interlayer spacing of the KN-CDs was calculated as 0.39 nm using the (002) plane. Additionally, the higher interlayer spacing of the KN-CDs (0.39) compared to that of bulk graphite (0.34 nm) is accredited to the presence of abundant functional groups at the edges of the KN-CDs [38]. Figure 1b depicts the Raman spectra of the KN-CDs with two distinct peaks at~1375 and 1580 cm −1 as the D-and G-bands, respectively [39][40][41]. The G-band (sp 2 -carbon) concerns the graphitization of the KN-CDs and the D-band (sp 3 -carbon) the defects of the KN-CDs [42]. The intensity ratio of the D-and B-bands (I D /I G ) was calculated as 0.62 and 0.67 for KN-CDs 1 and KN-CDs 2, respectively. These results of Raman studies suggest the moderate graphitization of the KN-CDs. The functional groups of the KN-CDs were characterized by FTIR spectroscopy ( Figure S1). The absorption bands around 3450-3200, 2930, and 2865 cm −1 correspond to the stretching frequency of O-H/N-H and asymmetric and symmetric C-H functional groups in the KN-CDs, respectively [43,44]. The absorption bands at 1770-1700, 1580, and 1400 cm −1 could be assigned to the stretching of C=O/N-H, C=C-C, and C-N/O-H(bending), respectively [45,46]. The absorption bands around 1275 and 1050 cm −1 suggest the presence of C-OH and C-O-C groups in the KN-CDs, respectively [46][47][48]. These FTIR results confirmed the presence of acid, hydroxyl, amino, and carbonyl moieties in the KN-CDs. Further, the functional groups and elemental compositions of the KN-CDs were examined by XPS techniques. Figure 2 displays the survey spectrum of the synthesized KN-CDs and its deconvoluted XPS spectra of C 1s, N 1s, and O 1s. Figure 2a reveals the presence of elements such as carbon, nitrogen, and oxygen with atomic percentages of 78/78, 2/4, and 20/18% in the KN-CDs 1/KN-CDs 2 at 284, 400, and 532 eV, respectively. Figure 2b,c displays the deconvolution spectrum of KN-CDs 1 and KN-CDs 2, respectively, at the C 1s level, which shows five peaks at 284.7, 285.5, 286.5, 287.6, and 289.0 eV ascribed to C-C/C=C, C-OH, C-N-C/C-O-C, C=O, and O=C-OH, respectively [49][50][51][52]. Figure 2d shows the high-resolution deconvolution spectrum of KN-CDs 1 at N 1s, revealing three peaks at 398.9, 400, and 401.9 eV, which correspond to the functional groups C-N-C, C-N-H, and (C) 3 -N, respectively. Figure 2e shows the deconvolution spectrum of KN-CDs 2 at the N 1s level, revealing three peaks at 398.8, 400, and 401.3 eV, which are attributed to the functional groups C-N-C, C-N-H, and (C) 3 -N, respectively [29,52,53]. In Figure 2f, the high-resolution spectrum at the O 1s level reveals four peaks ascribed to functional groups C=O, C-OH, C-O-C, and O=C-OH at 531.5, 532.7, 533.5, and 535.0 eV, respectively, for KN-CDs 1 and at 531.2, 532.4, 533.3, and 534.4 eV, respectively, for KN-CDs 2 ( Figure 2g) [54][55][56]. A slight shift in peaks of KN-CDs 2 at the N 1s and O 1s levels (Figure 2e,g) might be due to the increase in nitrogen content. From the XPS studies, the presence of hydroxyl, amine, and carboxyl functional groups in the KN-CDs can be concluded, which supports the results of the FTIR analysis.
FESEM images ( Figure S2a,b) of the KN-CDs 1 reveal a smooth surface, which might be due to the aggregation of the smallest particle with monodispersity. The chemical compositions present in the KN-CDs 1 of C, N, and O were revealed from the EDX spectrum ( Figure S2c). The elemental mapping of KN-CDs 1 shown in Figure 3 reveals the even distribution of C, N, and O. Similar to KN-CDs 1, KN-CDs 2 ( Figure 4) also reveals the smooth surface and even distribution of C, N, and O ( Figure S3 and Figure 4). However, the N content in KN-CDs 2 is higher than in KN-CDs 1. This is because of the use of NH 4 OH during the synthesis of KN-CDs 2. Figure 5 shows the TEM images of the KN-CDs. The particles are in the range of 3.0-9.0 nm. The average diameters of KN-CDs 1 and KN-CDs 2 were calculated as 5.6 and 5.1 nm based on statistical analyses of 50 carbon dots using Gaussian fitting (Figure 5a,d), respectively. The images reveal the near-spherical shape and monodispersed particles of the KN-CDs. The HRTEM images visibly display the lattice fringes, which confirms the fair graphitic nature of the KN-CDs. The d-spacing values were calculated as 0.21 nm attributed to the C(100) plane of graphitic carbon [31,57]. This acceptable graphitic nature of CDs shows consistent results with HRTEM, XRD, and Raman studies.   Figures 6a and 7a show the UV-vis spectra of KN-CDs 1 and KN-CDs 2, respectively. The KN-CDs exhibit two peaks at 275 and 327 nm indicating the π-π* transition of C=C and the n-π* transition of the C=O/C-N groups, respectively [58,59]. KN-CDs 2 shows a sharp peak of π-π* transition compared to KN-CDs 1. This sharp peak might be of higher nitrogen content in KN-CDs 2. The synthesized aqueous KN-CDs 1 and KN-CDs 2 solutions showed a transparent pale-yellow dispersion under daylight and emitted a cyanblue fluorescence under 365 nm UV light, which is revealed from the photographic images in Figures 6a and 7a. The fluorescence spectra of KN-CDs 1 and KN-CDs 2 are depicted in Figures 6b and 7b, respectively. Both KN-CDs 1 and KN-CDs 2 display excitation and emission spectra with an optimum excitation wavelength around 360 nm and an emission wavelength around 432 nm with good quantum yields of 14 and 19%, respectively (calculated using Equation (S1)). The fluorescence spectra of KN-CDs 1 and KN-CDs 2 at various excitation wavelengths are depicted in Figures 6c and 7c, respectively. As a result, the spectra clearly show the unique excitation-dependent emission spectra. The intensity of the emission spectra increased with the excitation wavelength from 300 nm to 360 nm. Then, the emission intensity decreased gradually with excitation wavelengths of 370 nm until 450 nm. The normalized excitation-dependent emission spectra of KN-CDs 1 and KN-CDs 2 are displayed in Figures 6d and 7d, respectively. Figure 6c,d, and Figure 7c,d suggest the red shift of the emission spectra with the increment of excitation wavelength. These results suggest the excitation-dependent emission spectra of the KN-CDs. This might be due to the surface state such as functional groups in KN-CDs, the quantum confinement effect, and the molecular fluorescence of the KN-CDs [58,60,61]. The above measurement result suggests the presence of high nitrogen content in KN-CDs 2, and thus, it is believed that because of high functionalities in KN-CDs 2 (Figure 7), the intensity of the emission spectra of KN-CDs 2 is higher than that of KN-CDs 1 ( Figure 6) and hence showed higher quantum yield.    Further, the stability of the KN-CDs solution was studied with storage time and irradiation time (Figure 8). Figure 8a,c shows the emission spectra of the KN-CDs (KN-CDs 1 and KN-CDs 2) at different storage times. Intensities are almost the same and display insignificant changes even after 90 days. In addition, the intensities did not change even after continuous UV light irradiation for 60 min. Photobleaching nor coagulation was not observed for longer storage time and under continuous irradiation of UV light. The photobleaching stability might be due to the electrostatic repulsions between the KN-CDs in solution [29,31]. Moreover, this reveals the photostability of the KN-CDs and thus suggests the utilization of the KN-CDs as metal ion-selective probes. Due to the difference in nitrogen content in KN-CDs 2 compared to KN-CDs 1, it was expected that there would be a difference in selection towards metal ions.
The prepared KN-CDs were used as metal ion sensors, which is usually detected by the interactions between metal ions and functional groups in the KN-CDs that form the complex. The change in emission spectra of the KN-CDs 1 and KN-CDs 2 solutions with various metal ions Al 3+ , Ca 2+ , Cd 2+ , Co 3+ , Cr 3+ , Cu 2+ , Fe 3+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+ , and Zn 2+ are depicted in Figures 9a and 10a, respectively. The intensities of emission spectra of the KN-CDs with and without metal ions show insignificant changes, except for Fe 3+ ions. The fluorescence intensities of the KN-CDs solution with Fe 3+ ions are nearly zero, suggesting that the quenching could be assigned for the possible complex formation. This result indicates that the prepared KN-CDs can be employed as specific, efficient, and selective sensors towards Fe 3+ ions due to the stronger affinity. Figures 9b and 10b show the histogram of the changes in the fluorescence intensities (F-F 0 /F 0 ) × 100 at 432 nm that occurred with various metal ions at the same concentrations (1000 µM). Aqueous KN-CDs 1/KN-CDs 2 and KN-CDs 1+Fe 3+ /KN-CDs 2+Fe 3+ solutions are pale-yellow and yellowishorange, respectively, under daylight. Further, they emitted cyan-blue fluorescence and turn-off fluorescence, respectively, under 365 nm UV light, as shown in the photographic images ofFigures 9b and 10b. This supports the fluorescence quenching of aqueous KN-CDs 1/KN-CDs 2 solutions in the presence of Fe 3+ ions. As shown in Figures 9c and 10c, the emission intensities decreased with the concentration (5-25 µM) of the Fe 3+ ions. Further, Figures 9d and 10d reveal a good linear relationship between (F-F 0 )/F 0 and the concentrations in the range of 5-25 µM of the Fe 3+ ions with R 2 of 0.998 and 0.999 using KN-CDs 1 and KN-CDs 2, respectively. The values of the limit of detection (LOD) for KN-CDs 1 and KN-CDs 2 were calculated as 0.95 and 0.85 µM, respectively, using Equation (1) [36].

Conclusions
In this work, a cheap, simple, and green method for the synthesis of KN-CDs (KN-CDs 1 and KN-CDs 2) using leftover kiwi fruit peel by the hydrothermal-carbonization method is reported. The existence of heteroatoms as chemical compositions in the KN-CDs was confirmed from FTIR, XPS, and EDX mapping studies. KN-CDs 1 and KN-CDs 2 exhibit a moderate degree of graphitization and narrow size distribution with average grain sizes of ca. 5.1 and 5.6 nm, respectively. Further, as-synthesized KN-CDs 1 and KN-CDs 2 emitted strong blue fluorescence under a UV light with quantum yields of 14 and 19%, respectively. Additionally, a redshift in emission with the excitation wavelength was observed. The fluorescence of the KN-CDs was prominently quenched with Fe 3+ ions and showed good linearity between concentrations of Fe 3+ in the range of 5-25 µM and (F 0 /F)/F 0 . The detection limits of Fe 3+ ions by KN-CDs 1 and KN-CDs 2 were obtained as 0.95 and 0.85 µM, respectively. Therefore, as-synthesized KN-CDs can be used as effective fluorescence probes for the detection of Fe 3+ ions with high sensitivity and selectivity in an aqueous medium without any further surface passivation. Thus, the unique KN-CDs from the natural waste material will motivate researchers towards the use of the KN-CDs for human safety and environmental pollution.