Graphene Quantum Dots Incorporated UiO-66-NH 2 Based Fluorescent Nanocomposite for Highly Sensitive Detection of Quercetin

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Introduction
Quercetin is the most important flavonoid in fruits and vegetables [1]. It does not produce in human bodies [2]. Quercetin is widely reported for antioxidant, antiviral, immunomodulation, antitumor [3], and anti-inflammatory [4] applications. The literature claimed that 945 mg/m 2 is the safe dose for quercetin. A high dose of quercetin can produce different several health issues including hypertension, a decline in potassium levels in serum, and emesis [2]. Therefore, accurate measurement of the concentration of quercetin is essential in the biomedical field [3]. Moreover, to measure the bioavailability of quercetin, it is essential for pharmacological response [1]. In general, analysis of quercetin with a simplistic, speedy, highly selective, and sensitive method is a prime necessity [4].
Nanomaterial-based recognition has earned tremendous consideration from the research community for the construction of sophisticated biomedical applications such as sensor design [12]. Presently, different types of sensing systems have been revealed for the recognition of quercetin including electrochemical sensors [3,8], fluorescence base sensors [13], etc. In this shade, nanoflakenanorod tungsten disulfide [2], gold nanoparticlesgraphene composite [11], silica gel-mediated carbon paste electrodes [4], multi-wall carbon nanotubes (MWCNTs) modified glassy carbon electrode (GCE) [14], silver-silica-based polyethylene glycol hybrid nanoparticles [9], carbon nanotube (CNT) modified electrode [7], etc., have been used for monitoring of quercetin in complexed samples. Herein, carbon-based fluorescence sensors have been widely preferred for sensing target analytes owing to their plenty of merits including speedy identification, cost-effectiveness, simplicity, high sensitivity, and selective detection capability [15]. Out of several carbon-based materials, fluorescent graphene quantum dots (GQDs) gained much attention from budding researchers in diverse fields including biosensing, chemical sensing, drug delivery, bioimaging, etc [15,16]. Principally, it consists of nanometer-sized materials made of single/multilayered graphenes. It has been divulged that light emission has size-based band gaps [17]. In addition, GQDs offer good biocompatibility, tunable photoluminescence, water-solubility, lower cost, and low toxicity [13,18]. Different GQDs-based fluorescence sensors have been documented including gold nanoparticle-GQDs-mediated nanozyme [19], sulfur-doped GQDs [13], MoS 2 -CNTs@GONRs/HS-CD/GQDs composite [20], etc. Despite this, there are indeed significant concerns with sensitivity and selectivity toward an intended target in the specimens presented.
Metal-organic frameworks (MOFs) have been used to capture biomarkers [21] and chemical ions [22,23]. Such nanostructures offer high porosity, larger surface area, surface tunability, etc [24]. Because of their distinct and adaptable features, luminous MOFs have recently been recognized for sensing applications among a plethora of MOFs [25]. Overall, the properties of MOFs that promise luminous inorganic frameworks represent substantial benefits above other conceivable kinds of sensing elements. Furthermore, the logical creation of such structures has emerged as the key objective in the investigation of MOFs as detecting materials [26]. Mainly, zirconium and 2-aminoterphthalic acid (BDC-NH 2 or 2-ATA)-mediated UiO-66-NH 2 (UiO-66: Universitetet i Oslo) have gained huge consideration from researchers for sensing applications possibly because of their hopeful chemical and physical characteristics [27]. MOFs have several drawbacks, including structural collapse and fluorescence intensity [28]. Several investigations reported the incorporation of GQDs into MOFs for measurement applications wherein GQDs can be uniformly dispersed and distributed in MOFs [17]. The functionality of such GQDs@MOFs-based fluorescent nanosensors is offered in aspects of responsiveness, specificity, measurement speed, etc [29]. Principally, it may be because of the synergistic presentation of GQDs and luminescent MOFs [29,30]. According to this research, the spongy framework of UiO-66-NH 2 may encourage the inclusion of nanosized GQDs, resulting in improved performance [26]. Despite plenty of advancement in sensing quercetin, a highly luminescent GQDs@UiO-66-NH 2 nanoprobe is still missing for the detection of quercetin. As a result, GQDs-adorned MOF-based fluorescent nanoprobes might be employed to measure quercetin.
The present study aims to construct a bright blue luminescent GQDs@UiO-66-NH 2 based "Turn-Off" nanoprobe for the sensing of quercetin ( Fig. 1) with enhanced effectiveness in terms of responsiveness, specificity, detection speed, involved cost, manufacturing, sensing tactic, etc., that contrasted to initially disclosed detectors. Green synthesis of GQDs was obtained from green precursor using hydrothermal method, whereas synthesis of fluorescent luminescent UiO-66-NH 2 was prepared using 2-ATA as a ligand and zirconium (Zr) as metal ions source. The sensing of quercetin in phosphate buffer (pH=7.4) was studied, whereas selectivity analysis was performed using different interfering substances. As a consequence, the suggested luminescent GQDs@UiO-66-NH 2 nanoprobe affords a straightforward, environmentally, cost-effective, quick, highly specific, and delicate function for quercetin identification. As a corollary, this fluorescence detector will enable a different option for monitoring flavonoids in clinical specimens.

Green synthesis of GQDs
In this study, the hydrothermal approach was used to produce exceptionally bright blue fluorescent GQDs from cornhusks (Fig. 2). Initially, obtained 100 g of cornhusk was trimmed into tiny chunks. Then, it was powdered in a laboratory crusher for further use. 50 g of ground husk fibers was then diffused in an ethanol-water combination with a ratio of 20∶40 for 2 days at room temperature to eliminate any pollutants and dust. After a couple of days, the corn husk fibers were withdrawn from the aforesaid mixture and heated for 1 h in a laboratory hot air oven (Bio-Technics, India) at 100 °C before being triturated. Following that, 3 g of fibers was poured into 40 mL of DDW, which was then constantly stirred at 100 r/min for 15 min with a laboratory magnetic stirrer at room temperature. After this, to generate GQDs, the solution was poured into a Teflonlined autoclave in a stainless-steel hydrothermal vessel and housed in a 160 °C laboratory oven for 12 h [15]. The solution was cooled to room temperature when the hydrothermal phase was accomplished. The color of the solution changed from yellow-white to yellowish-brown during this phase, likely due to the cornhusk. The resulting GQDs were then filtered using 0.22 μm pore size membrane filter paper and freeze-dried (Southern Scientific Lab Instrument, Chennai, India) following the procedure from Ref. [15]. In the first step, green synthesized GQDs were subjected to the primary freezing process wherein concentrated GQDs were made at 30 °C for 12 h using a deep freezer. The frozen GQDs solution was then freez-fried for 24 h at -53 °C and under the pressure of 1.6 Pa. After the primary drying of GQDs, secondary drying was performed to remove the remaining moist content from GQDs powder. Herein, the temperature was maintained at 10 °C for 8 h, and then increased up to 25 °C for 4 h. Subsequently, the temperature continuously rose with a rate of 1 °C/min. To complete the drying phase, the temperature of the cold trap end was tuned to -53 °C. The green-produced GQDs-free dried powder was then tested for several spectroscopic analyses.

Synthesis of MOFs
A formerly published simple methodology was utilized for the production of Uio-66-NH 2 with slight modification [31]. First, 0.348 g of ZrCl 4 was dissolved in an exact volume of 65 mL of DMF, and then 0.276 g of 2-ATA was added into a solution of ZrCl 4 , followed by sonication for 20 min at 25 °C. The solution was then placed in a teflon-lined autoclave over 120 °C for 24 h to accomplish the manufacturing of intensely luminous Uio-66-NH 2 . Thereafter, the autoclaved mixture was treated to cold centrifugation at 20 000 r/min for 25 min at 25 °C (Elteck Overseas Pvt., India) to isolate Uio-66-NH 2 . At this stage, Uio-66-NH 2 was washed with the ethanol, and then again washed with DDW in triplicate to eliminate contaminants. Furthermore, the resulting Uio-66-NH 2 was freeze-dried following the earlier described freeze-drying process [15]. To evaluate the effective synthesis of MOF, freezedried Uio-66-NH 2 was analyzed using several spectroscopic methods.

Fabrication of GQDs@UiO-66-NH 2 nanoprobe
Highly luminous GQDs@UiO-66-NH 2 nanoprobe was developed using the previously discussed solvothermal approach [30] with slight modification. To begin with, 10 mg of freeze-dried GQDs were transferred to a clean 50-mL volumetric flask (200 μg/mL), followed by volume adjustment with DDW, and then sonication for 15 min at 30 °C. Later, 2 mg of UiO-66-NH 2 powder was poured into a cleansed 10-mL volumetric flask, and the volume was corrected with DDW. Following that, different concentrations of UiO-66-NH 2 were prepared to obtain the suitable concentration for the fabrication of a highly fluorescent GQDs@UiO-66-NH 2 nanoprobe for sensing quercetin. Herein, an optimum concentration of UiO-66-NH 2 was added into the previously prepared GQDs solution and following that, transferred to bath sonication for 1 h at 30 °C. The resulting combination of GQDs and UiO-66-NH 2 was then moved to the autoclave and steamed (120 °C over 24 h). The solid phase of the nanosensor was then segregated out from the free form of GQDs employing cold centrifugation around 20 000 r/min for 30 min at 25 °C. Subsequently, washing was performed using DDW in triplicate. Last, the prepared highly luminous GQDs@UiO-66-NH 2 nanoprobe was subjected to freeze-drying using the previously reported method and then further used for spectroscopical characterizations and sensing of quercetin.

Sensing of quercetin
A highly fluorescent GQDs@UiO-66-NH 2 nanoprobe was employed to assess quercetin with vast sensitivity. First, 100 μg/mL previously prepared GQDs@UiO-66-NH 2 nanoprobe was subjected to fluorescence measurement. Then, different concentrations of 50-500 ng/mL of quercetin were individually prepared using phosphate buffer (pH = 7.4, 10 mmol/L) in cleaned 5-mL volumetric flasks. For the sensing investigation, produced quercetin concentrations were digested with the GQDs@UiO-66-NH 2 nanoprobe for 15 min to enable the reactivity of the GQDs@UiO-66-NH 2 nanoprobe. After that, the quenched fluorescence of the prepared GQDs@UiO-66-NH 2 nanoprobe was measured via a spectrofluorometer. The linear concentration and range and limit of detection (LOD) for quercetin were measured. Herein, ∆F was calculated using the ratio of fluorescent intensity of the probe in the nonattendance (F 0 ) and attendance (F) of quercetin concentrations. After that, LOD was assessed via slope (m) and standard deviation (σ), and the limit of quantification (LOQ) was measured:

Other analytical parameters
To confirm the selectivity aptitude of constructed GQDs@UiO-66-NH 2 nanoprobe for quercetin, several metal ions, amino acids, and proteins were preferred as interfering agents mainly glucose, sodium chloride, potassium chloride, bovine serum albumin, lysine, and quercetin. In brief, the same concentration of each interfering substance and quercetin were prepared in a separate volumetric flask (5 mL) using phosphate buffer (pH = 7.4). For investigation, 5 mL of GQDs@UiO-66-NH 2 nanoprobe solution (100 μg/mL) was added into a separate test tube and then 1 mL of individual interfering substance was added (200 ng/mL). After that, this solution was allocated for 5 min to enable the association between the nanoprobes and interfering material. The fluorescence intensity of the GQDs@UiO-66-NH 2 nanoprobe was then evaluated, and the same tests were carried out for all interfering compounds and quercetin. In addition, a mixture of all interfering substances (sample Q) with quercetin was crosschecked to investigate the combined effect on the fluorescence potential of the fabricated nanoprobe. In addition, other essential parameters including precision, stability, and repeatability were measured to confirm the practical applicability of the designed fluorescent sensor.

UV-Vis spectroscopy
The UV-Vis absorption spectra of GQDs, UiO-66-NH 2 , and GQDs@UiO-66-NH 2 nanoprobe are displayed in Fig. 3. In essence, the UV-Vis spectra of the obtained GQDs solution demonstrated a high absorption point at 276 nm that ascribed to the π→π * transition (C＝C bond). In addition, a shoulder point at 340 nm is ascribed to n→π * transition (C＝ O bond). Hence, it departs GQDs from carbon dots (CDs) [32]. As a result, it assured the effective manufacturing of GQDs from organic substrates. The UV-Vis absorption spectra of UiO-66-NH 2 featured two absorption maxima, one at 273 nm and the other at approximately 365 nm. The first peak point at 273 nm corresponds to ligand-to-metal charge transfer (LMCT). The second peak spike at 365 nm is attributable to the interactions of amino groups containing lone pairs of electrons with the benzene ring's π* orbital [33]. Furthermore, the absorption band of GQDs@UiO-66-NH 2 conjugate displayed a wide absorption peak at the center of 340 nm and an absorption sort of 300-500 nm, showing the combined absorption (overlapping) of GQDs and UiO-66-NH 2 . Therefore, it assured the synthesis of GQDs and UiO-66-NH 2 -mediated GQDs@UiO-66-NH 2 nanoprobe.

UV cabinet fluorescence study
After proof of UV-Vis spectroscopy, the fluorescent analysis was confirmed using a UV cabinet. Figure 4 demonstrated the UV cabinet-based analysis of GQDs, obtained UiO-66-NH 2 , and GQDs@UiO-66-NH 2 nanoprobe at different wavelengths. After completion of the hydrothermal reaction, the color of the precursor powder suspension changed from yellowish-brown to dark brown, which marked the green synthesis of GQDs. Herein, GQDs were observed in a laboratory UV cabinet ( Fig. 4(a)) that shows yellowish-brown in visible light whereas green fluorescence ( Fig. 4(b)) under λ max = 254 nm light and bright blue fluorescence under UV light λ max = 365 nm (Fig. 4(c)). Consequently, green synthesized GQDs had strong blue fluorescence in longer UV wavelengths which may be because of electronic energy transition [34]. Overall, it confirmed the accomplishment of GQDs from a green precursor. For UiO-66-NH 2 , fluorescence ability was assessed in a laboratory UV cabinet. Figure 4 furnishes creamish white in visible light (Fig. 4(d)) while white fluorescence (Fig. 4(e)) under λ max = 254 nm light and bright blue fluorescence under UV light λ max = 365 nm (Fig. 4(f)). As a result, it confirmed that UiO-66-NH 2 had strong blue fluorescence in longer UV wavelengths which may be because of 2aminoterephthalic acid as a linker having fluorescence properties [33]. In addition, the GQDs@UiO-66-NH 2 nanoprobe was subjected to fluorescence study using the laboratory UV cabinet (Fig. 4). It shows creamy white in visible light ( Fig. 4(g)) whereas white fluorescence (Fig. 4(h)) under λ max = 254 nm light and bright blue fluorescence under UV light λ max = 365 nm ( Fig. 4(i)). As a result, it confirmed that UiO-66-NH 2 had strong blue fluorescence in longer UV wavelengths than bare UiO-66-NH 2 and GQDs. It's probable that the elevation in conjugate fluorescence is attributable to the fluorescence of GQDs and UiO-66-NH 2 (synergistic effect takes place).

Excitation and emission spectrum of GQDs
The optical characteristics of green synthesized GQDs were investigated using fluorescence excitation and emission spectroscopy in this work. 345 nm and emission peaks around 455 nm, respectively [35].

Particle size analysis
The particle size analysis of GQDs, UiO-66-NH 2 , and GQDs@UiO-66-NH 2 is illustrated in Fig. 6. It has been asserted that particle size has the greatest consequence on optical qualities and surface stability [36]. Green-produced GQDs had a particle size of 43.5 nm (Fig. 6(a)), confirming the generation of nanosized GQDs from cornhusk. The polydispersity index (PDI) of green synthesized GQDs was determined to be 0.145, indicating that GQDs in solution is distributed uniformly. The mean particle size of UiO-66-NH 2 was measured to be 58.8 nm (Fig. 6(b)), and the PDI was 0.319, indicating homogeneous nanosize particle distributions in formed dispersion. The averaged particle size of the intensely fluorescent GQD@UiO-66-NH 2 nanoprobe was 89.4 nm (Fig. 6(c)), and the polydispersity index was 0.167, confirming homogeneous conjugate distributions. The enhancement in the size of the nanoprobe could be because of the incorporation of MOFs and GQDs. The HR-TEM study of GQD, UiO-66-NH 2 , and GQD@UiO-66-NH 2 nanoprobe were undertaken for additional confirmation.

Zeta potential analysis
The zeta potential is a significant parameter that affects a solution's stability. In this line, the literature survey reported that particles having a higher positive or negative zeta potential are thought to produce a stable solution [37]. The surface charge of greenproduced GQDs was -27.29 mV, revealing that GQDs are durable in solution ( Fig. 7(a)). Moreover, the negative zeta of GQDs assured oxygen-based surface functionality such as hydroxyl, carboxyl, and epoxy [38]. The surface charge of UiO-66-NH 2 was observed to be +30.02 mV, which assured the stability of MOF in the solution (Fig. 7(b)). It shows the potential that may be because of the amine group on the UiO-66-NH 2 MOFs surface [39]. Finally, the surface charge of the fabricated GQD@UiO-66-NH 2 nanoprobe was +24.75 mV, which assured the good stability of the nanoprobe in the solution (Fig. 7(c)).
Herein, there is a change in the zeta potential of nanoprobe as compared to the bare UiO-66-NH 2 MOFs and GQDs, which may be because of the conjugation/masking of surface functionality of GQDs into porous UiO-66-NH 2 .

FTIR spectroscopy
In this step, the FTIR spectroscopy verified the hydrophilicity of green synthesise GQDs ( Fig. 8(a)).
In summary, O−H stretching vibration, C−H stretching vibration, C＝ O stretching vibration, and C−O stretching vibration were discovered to be at 3 313, 2 939, 1 638, and 1 030 cm -1 , respectively. Consequently, the existence of carboxylic functionality on the exterior of green-produced GQDs was verified. Figure 8(b) demonstrated the FTIR spectra of fluorescent UiO-66-NH 2 . Briefly, peaks at 3 356 and 3 259 cm -1 confirmed the occurrence of amine (N−H) stretching. Herein, the overlapping of primary amines of UiO-66-NH 2 and OH of water molecules present in the powder of UiO-66-NH 2 resulted in a wide peak between the regions of 3 250-3 380 cm -1 . The strong intense peak point at 1 590 cm -1 indicated the presence of C＝O stretching. The intense peak at 1 239 cm -1 confirmed the occurrence of C−N functionality. In addition, C−O stretching vibrations were obtained at 1 375 cm -1 [40]. Simply put, it validated the fabrication of UiO-66-NH 2 MOFs employing the recommended linker and metal ion origin. Figure 8(c) displayed FTIR of prepared luminescent GQDs@UiO-66-NH 2 nanoprobe. In this, different UiO-66-NH 2 MOFs peaks include main Zr−O stretching vibration, and symmetric/asymmetric N−H stretching vibration were observed around 400-650 and 3 356-3 259 cm -1 , respectively. Also, the bonding between aromatic carbon and nitrogen (C−N) was seen at 1 239 cm -1 . Moreover, the UiO-66-NH 2 −MOFs demonstrate carboxylic stretching vibrations at 1 590 cm -1 whereas other FTIR peaks including C−O stretching vibrations, and chloride stretching vibrations were obtained at 1 018 and 636 cm -1 , respectively. The FTIR peaks of −OH and −NH 2 were obtained at 3 050-3 600 cm -1 that assured the presence of primary amine in GQDs@UiO-66-NH 2 -MOFs. In addition, a broad FTIR peak at 3 100-3 500 cm -1 indicates the existence of water molecules in porous 3 050-3 600 cm -1 (Zr−OH). Finally, the FTIR peak shifts reported the existence of hydrogen bonds across the main amine and hydroxyl units. Furthermore, certain GQDs typical spikes at around 2 900 cm -1 have emerged. It guaranteed the fabrication of GQDs@ UiO-66-NH 2 MOFs derived nanoprobes from naked GQDs and UiO-66-NH 2 MOFs [41].

HR-TEM analysis
The morphology and size of prepared nanosized GQDs, bare UiO-66-NH 2 , and GQDs@UiO-66-NH 2 nanoprobe were confirmed using HR-TEM. Figure 9(a) depicts the HR-TEM image of green synthesized GQDs wherein it shows the uniform size distribution with 20 nm in an average diameter. Overall, it assures the synthesis of nanosize and uniform distribution of GQDs from green precursors. Figure 9(b) depicts the HR-TEM image of UiO-66-NH 2 . It shows the regular octahedron nanostructure with 100 nm of average particle size [27]. Figure 9(c) depicts the HR-TEM image of the final GQDs@UiO-66-NH 2 probe. It shows the successful encapsulation of GQDs into UiO-66-NH 2 . That may be because of the porous nature of MOFs and weak interactions with the NH 2 functionality of UiO-66-NH 2 and the carboxyl functionality of GQDs. As a result, it demonstrated that the average particle size of 10 nm confirmed that slight disruption of the structure of MOFs may be an encapsulation of GQDs in the pores of MOFs. To summarise, the GQDs@UiO-66-NH 2 probe was prepared utilizing green-generated GQDs and UiO-66-NH 2 .

Sensing of quercetin
In this step, the optimization of a suitable concentration of synthesized UiO-66-NH 2 was accomplished using numerous concentrations with green synthesized GQDs. Herein, the addition of prepared concentration of MOFs into GQDs shows the boosting of fluorescence intensity of conjugate. Herein, the concentration of MOFs shows a proportional relationship with fluorescent intensity augmentation. Finally, 140 μg/mL UiO-66-NH 2 was obtained as a suitable concentration for fabrication of GQDs@UiO-66-NH 2 based fluorescence nanoprobe for detection of quercetin. After the addition of other next concentrations, it does not demonstrate the proportional relationship which may be because of the complete conjugation of GQDs with bare UiO-66-NH 2 . It produced more fluorescence intensity over naked GQDs and UiO-66-NH 2 . The optimized concentration-based GQDs@UiO-66-NH 2 probe was further subjected to the sensing study of quercetin. In this study, sensing of quercetin using a fabricated GQDs@UiO-66-NH 2 probe was depicted in Fig. 10. The addition of varied prepared quantities of quercetin culminated in the dampening of the nanoprobe's fluorescence referred to as fluorescence "Turn-Off". Herein, conjugation of GQDs and UiO-66-NH 2 probe resulted in the high bright blue  fluorescence. In addition, it has been divulged that the photosensitivity of UiO-66-NH 2 was boosted with the conjugation of green synthesized GQDs [31]. After the addition of quercetin from 50 to 500 ng/mL, it illustrates the fluorescence quenching ( Fig. 10(a)). It might be due to UiO-66-NH 2 having a higher binding affinity and adsorption potential for quercetin. Possibly, it is because of the complexation of zirconium ions with 3-OH and 4-C＝O functionality of quercetin. The calibration curve of quercetin using GQDs@UiO-66-NH 2 nanoprobe was provided in Fig. 10

Anti-interference potential of GQDs@UiO-66-NH 2 nanoprobe
The anti-interference aptitude of fabricated GQDs@UiO-66-NH 2 nanoprobe was measured using a different interfering agent. The fluorescence intensity of the produced fluorescent nanoprobe was reduced by a minor amount in the presence of other chosen interfering chemicals. Importantly, it might be because UiO-66-NH 2 has a higher binding affinity and adsorption capability for quercetin than the other interfering agents. Furthermore, a combination of all interfering compounds quenched the fluorescence of the nanoprobe slightly more than pure quercetin in the nanoprobe. Overall, it confirmed the anti-interference potential of GQDs@UiO-66-NH 2 nanoprobe towards the quercetin in the occurrence of several biomolecules and ions. After this, GQDs@UiO-66-NH 2 precision was reported here based on interday (n = 6) and intraday (n = 6) output. In consequence, the relative standard deviation (RSD) was determined  to be 1.20% for interday and 3.56% for intraday, respectively. The repeatability of the projected final GQDs@UiO-66-NH 2 nanosized sensor was then determined using 300 ng/mL of quercetin, resulting in RSD of 2.90% (less than 5%). It ensured that the suggested sensor for measuring quercetin is repeatable. Finally, the suggested sensor's stability was tested for 10 days at 25 °C. The RSD was determined to be 4.20% (less than 5%), indicating that the sensor is stable for up to 10 days under testing circumstances.

Conclusion
The present work reported the GQDs@UiO-66-NH 2based fluorescence "Turn-On-Off" nanoprobe for the detection of quercetin. In conclusion, nanosize stable GQDs were obtained from corn husk whereas UiO-66-NH 2 was synthesized using 2-ATA as an organic linker and zirconium as a metal ions source via the hydrothermal method. The significant increment in fluorescence of nanoprobe to the bare GQDs and UiO-66-NH 2 assured the synthesis of a high fluorescent sensor. The UV-Vis spectroscopy confirmed the formation GQDs, UiO-66-NH 2, and GQDs@UiO-66-NH 2 . The zeta potential of GQDs, UiO-66-NH 2, and GQDs@UiO-66-NH 2 assured the stability of the prepared nanoprobe. The HR-TEM images signify the distribution of GQDs in the porous structure of UiO-66-NH 2 without any structural collapse. Finally, the sensing study shows that the addition of quercetin into the nanoprobe shows the quenching of bright fluorescent and provides a wide concentration range.
Herein, owing to the towering affinity of porous UiO-66-NH 2 , it shows the response to quercetin. Probably, the complexation of zirconium ions with 3-OH and 4-C＝ O functionality of quercetin may responsible for decrease in fluorescence intensity. The further analytical features including stability, repeatability, selectivity, and precision assured the development of an ideal sensor for the detection of quercetin. Overall, this nanosize design of fluorescence-based GQDs@UiO-66-NH 2 nanoprobe can be used as an outstanding alternative for sensing quercetin in biological samples and other various analytical purposes owing to their facile, speedy, cost-effective, eco-friendly, highly sensitive, and selective sensing ability.