Terbium-based dual-ligand metal organic framework by diffusion method for selective and sensitive detection of danofloxacin in aqueous medium

A water-dispersible Tb(III)-based metal organic framework (TBP) was produced by diffusion technique using benzene-1,3,5-tricarboxylic acid (BTC) and pyridine as easily accessible ligands at low cost. The as-synthesized TBP with a crystalline structure and rod-shaped morphology has exhibited thermal stability up to 465 °C. Elemental analysis confirmed the presence of carbon, oxygen, nitrogen, and terbium in the synthesized MOF. TBP was used as a fluorescent probe for detection of danofloxacin (DANO) in an aqueous medium with significant enhancement of fluorescence intensity as compared to various fluoroquinolone antibiotics (levofloxacin (LEVO), ofloxacin (OFLO), norfloxacin (NOR), and ciprofloxacin (CIPRO)) with a low detection limit of 0.45 ng/mL (1.25 nm). The developed method has successfully detected DANO rapidly (i.e., response time = 1 min) with remarkable recovery (97.66–101.96%) and a relative standard deviation (RSD) of less than 2.2%. Additionally, TBP showcased good reusability up to three cycles without any significant performance decline. The in-depth mechanistic studies of the density functional theory (DFT) calculations and mode of action revealed that hydrogen bonding interactions and photo-induced electron transfer (PET) are the major factors for the turn-on enhancement behavior of TBP towards DANO. Thus, the present work provides the quick and precise identification of DANO using a new fluorescent MOF (TBP) synthesized via a unique and facile diffusion technique.


Introduction
Fluoroquinolone antibiotics (FQs), also referred to as quinolones, quinolone carboxylic acids, pyridine carboxylic acids, and 4-quinolones, are a large and rapidly growing class of synthetic antibacterial medicines (Giguère and Dowling 2013).In 1962, nalidixic acid marked the beginning of an important antibacterial drug family of antibacterial agents (Lesher et al. 1962).The earliest members of the fluoroquinolone family of antibiotics, such as norfloxacin, levofloxacin, ciprofloxacin, and ofloxacin, were introduced on the global pharmaceutical market nearly three decades ago (Charushin et al. 2014).FQs are formed by modifying the core structure of quinolones by introducing a fluorine group at C-6 and a piperazine derivative (or piperazinyl) group at C-7 (table S1) (Sukul and Spiteller 2007).These antibiotics are effective in treating a variety of bacterial infections, including skin, soft tissues, bones, and joint infections, as well as respiratory tract and urinary tract Highlights • Dual-ligand MOF (TBP) was synthesized by diffusion method.
• TBP has shown excellent selectivity for fluorescence detection of DANO.
• Low detection limit (0.45 ng/mL) validates the excellent sensitivity of developed MOF.
• TBP has a significant ability to be reused without apparent loss of sensitivity.
• Fluorescent detection of DANO was successfully revealed by DFT study and H-bonding.
Responsible Editor: Angeles Blanco infections (Liu 2010).Also, FQs are ubiquitously found in surface waters after being released from hospitals or domestic wastewater from municipal areas (Fink et al. 2012).Since such FQs are not entirely eliminated at wastewater treatment plants, their continuous release into the environment makes FQs "pseudo-persistent" compounds.Through their intense adsorption on organic material and minerals, the FQs, which initially exist in water bodies, rapidly migrate into the soil and sediments (Frade et al. 2014).Various research groups expressed their major concern towards FQs due to their hazardous nature for both aquatic and terrestrial species.Robinson et al. investigated the toxic effects of seven FQs on various prokaryotic and eukaryotic aquatic species, including ciprofloxacin, lomefloxacin, ofloxacin, and levofloxacin (Robinson et al. 2005).FQs are toxicologically significant in animal test models at high doses, and there is some evidence that FQs can affect the growth and normal reproductive processes of fish, water fleas, macrophytes, amphibians, and green algae at low environmental doses when persistent exposure to active forms of these compounds occur (Janecko et al. 2016).
Nowadays, FQs can be determined with a variety of analytical procedures, including electrochemical sensors (Goyal et al. 2012), electrocatalytic oxidation and voltammetry (Zhang et al. 2014), capillary electrophoresis (CE) (Zhang et al. 2018), high-performance liquid chromatography (HPLC) (Han et al. 2011), surface-enhanced Raman spectroscopy (SERS) (He et al. 2010), enzyme-linked immunosorbent assay (ELISA) (Adrian et al. 2012;Xing et al. 2020), and liquid chromatography-tandem mass spectrometry (LC-MS/MS) (Li et al. 2020b).Despite the significant advantages of sensitivity and selectivity, these techniques are time-consuming, labor-intensive, and expensive to operate due to the need for specialized equipment, extensive pretreatment, and high operating costs.As a result, these methods are difficult to devise for on-site monitoring of FQs (Serebrennikova et al. 2020).Fluorescence detection thus offers a substitute for high selectivity, sensitivity, and accuracy, as well as low cost, quick, and easy operation for the detection of antibiotics.
Metal organic frameworks (MOFs) have attracted considerable attention and growth over recent years.A set of crystalline organic-inorganic hybrid materials build up from metal ions or metallic assembly acts as nodes interconnected by the organic ligands as a bridge are known as MOFs, which have shown a number of applications in various fields like sensing (Xiao et al. 2021), catalysis (Guo et al. 2023;Belmabkhout and Cordova 2023), drug delivery (Lawson et al. 2021;Parveen et al. 2023), bio-imaging (Wang 2017), magnetism (Ricco et al. 2013), photocatalysis (Liu et al. 2022;Saleem et al. 2023;Issa Hamoud et al. 2022), electrocatalysis (Ko et al. 2020), adsorption (Elsherbiny et al. 2023;Ahmad et al. 2020Ahmad et al. , 2022a, b;, b;Fallatah et al. 2022;Ayub et al. 2022), and gas storage/ separation (Li et al. 2018) due to their versatile properties such as well-defined crystal structure, high surface area, porosity, low density, high surface to volume ratio, high pore volume, and chemical tunability (Lu et al. 2014;Xiao et al. 2023).
A number of techniques for the production of MOFs have been proposed, including hydrothermal/solvothermal methods (Denisov et al. 2019), electrosynthesis (Zalpour et al. 2022), mechanochemical (Kaur et al. 2023), sonochemical (Vaitsis et al. 2022), and microwave synthesis (Forsyth et al. 2020).However, these have some drawbacks, such as the need for high temperatures and pressures, high consumption of energy, complicated processes, the need for sophisticated equipment, the introduction of unwanted anions when using metal salts, and difficulty in regulation.As a result, for the field to advance, a mild and clean synthetic approach to overcome the aforementioned drawbacks needs to be developed.So, the diffusion technique can be considered facile and environmentally benign, with advantages such as mild reaction conditions, simple operation, and a clean process.
In this research, a dual-ligand fluorescent terbium MOF (TBP) has been synthesized by a facile diffusion method using low-cost ligand precursors (BTC and pyridine).To the best of our knowledge, this study is the first detailed assessment of the quantitation of DANO by using TBP.The presence of specific functionalities with superior structural integrity of the TBP was found to be highly suitable for the sensing of DANO.DANO has been quickly and precisely recognized by TBP in the aqueous medium with a detection limit (LOD) of 0.45 ng/mL (1.25 nm).The proposed method can help improve the performance of sensors for the accurate analysis of various target analytes, especially antibiotics present in environmental and biological matrices.Theoretical results provide a full explanation for evidence of forward reaction steps and find a significant correlation between experimental and theoretical work.As a result, the current study is expected to shed more light on the chemical, structural, and spectroscopic features of synthesized compounds.

Measurements
On the Perkin Elmer RXIFT-IR Spectrophotometer (Japan), the Fourier transform infrared (FT-IR) spectra of the ligands and TBP were scanned in the 400-4000 cm −1 range.With the help of Panalytical's X'Pert Pro Powder X-ray Diffractometer [CuKα X-ray (λ = 1.54 Å), 45 kV, and 40 mA], PXRD diffractograms for the determination of crystal structure were acquired.The Brunauer-Emmett-Teller (BET) surface area analyzer (Microtrac Belsorp Mini-II, Bel, Japan, Inc.) was used for the measurement of N 2 adsorption-desorption isotherms.Morphological and topographical examinations of synthesized MOF were conducted using a Hitachi SU8010 Series (Japan) field emission scanning electron microscope (FE-SEM).An energy-dispersive X-ray (EDX) spectrometer was used to obtain the EDX spectrum supplied as a surplus FE-SEM accessory (Hitachi SU8010 Series, Japan) mounted firmly on the stub of the specimen.Thermogravimetric analysis (TGA) was performed on a Hitachi STA7300 (Japan) with a range of temperatures from 35 to 800 °C (heating rate = 10 °C/min) in the atmosphere of pure nitrogen.For photoluminescence (PL) studies, the Shimadzu RF-5301PC spectrofluorometer (Japan) was used.

General procedure for the synthesis of TBP
TBP was synthesized by adopting the diffusion technique (Scheme 1).Firstly, the ligand solution was prepared by mixing 42 mg (0.2 mmol) of BTC and 79 mg (1 mmol) of pyridine in ethanol (10 mL), and the metal solution was obtained by dissolving 87 mg (0.2 mmol) of Tb(NO 3 ) 3 .5H 2 O in water (10 mL).Both solutions were sonicated for 5 min.After that, metal solution (2 mL) was poured into a glass test tube, and then ligand solution (2 mL) was added to the same tube carefully along the sidewalls of the test tube.This resulted in the layer formation between metal and ligand solutions.Then, the glass tube was placed on a stand without disturbing it for 5 days to get a better formation of the MOF crystals.White crystalline solid particles were obtained near the middle layer.The product (TBP) was collected after centrifugation at 8000 rpm (10 min) and washing five times with ethanol and water (5 mL each) separately.Finally, the obtained MOF crystals were dried and shifted to a desiccator (anhydrous CaCl 2 as desiccant) to keep them in a moisture-free state till used for PL sensing studies of FQs.

Luminescent sensing
A stock suspension for photoluminescence analysis was prepared by dispersing 1 mg of excellently ground TBP in 100 mL of deionized water and sonicating it for 20 min.The photoluminescence sensing studies on the abovementioned stock suspension were carried out at room temperature.Because of its intriguing luminous behavior, the potential use of TBP as a photoluminescence sensing material for the identification of several FQs was investigated.The solutions of analytical standard FQs (LEVO, OFLO, NOR, CIPRO, and DANO) were prepared in water as a 3 ng/mL aqueous solution.Various concentrations of analyte solutions were made by dilution of a 3 ng/mL solution for a thorough investigation of the selective detection of FQs.A good selective analysis was done by taking 100 μL of TBP stock suspension mixed with 2.9 mL (3 ng/ mL) of given FQs to make the total volume up to 3 mL.For all luminescent measurements, the photoluminescence spectra were recorded using a spectrofluorometer with λ ex = 353 nm and slit widths of 5 nm and 3 nm for excitation and emission monochromators, respectively, by selecting the most intense peak.

Synthesis and characterization of TBP
TBP was synthesized using the diffusion technique with an equimolar amount of terbium nitrate pentahydrate and BTC (with an excess of pyridine).Scheme 1 depicts the overall synthesis process.

FT-IR
The presence of functional groups in TBP was confirmed using FT-IR determinations, as shown in Fig. 1.The characteristic bands have been observed for pyridine, i.e., 1437 and 1581 cm −1 corresponding to C=N and C=C, respectively.BTC has shown broad stretch for O-H at 2843 cm −1 and other stretching bands at 1694 and 1267 cm −1 resulting from the carbonyl of carboxylic acid and C-O in that order.Moreover, some new stretching bands have been found in TBP, i.e., 1610TBP, i.e., -1553TBP, i.e., , 1434TBP, i.e., -1370, 511, 511, and 457 cm −1 , belong to the stretching of the asymmetric and symmetric carboxylic groups, C=N, Tb-O, and Tb-N respectively.The absence of two major stretching bands in pyridine and BTC, i.e., Tb-O and Tb-N, but their presence in TBP confirms the appropriate bonding of both ligands with Tb 3+ in TBP.Such types of stretching frequencies have confirmed the formation of TBP.

PXRD
For the crystallographic investigation of TBP under the current experimental system, PXRD was performed (Fig. 2).In comparison to the relative positions of the peaks, the diffraction peaks observed for synthesized TBP at the 2θ scale matched closely with the standard diffraction pattern obtained for the Tb-MOF synthesized by stirring method (Bhardwaj et al. 2016).This result reveals that the TBP synthesis was successful.The recorded PXRD diffractograms showed sharp peaks at 9. 98, 10.26, 13.52, 17.45, 17.96, 20.56, and 31.09,having miller indices (100), ( 010), ( 001), ( 110), ( 101), (111), and ( 210) respectively which demonstrated the crystalline structure of the synthesized TBP.The average crystallite size (D) was estimated with the help of the Debye-Scherrer equation (D = Kλ/βcosθ, where K is the shape factor having value 0.89, λ denotes the X-ray wavelength having value 0.15406 nm, β is the full width at half maxima (FWHM), and θ denotes the Bragg's diffraction angle), and it was found to be 46 nm.

TGA
The TGA was applied to examine the thermal stability of the synthesized TBP (Fig. 3).According to the first resulting curve, approximately 21.97% mass reduction occurred in the range of 50-115 °C, which is related to processes of dehydration and de-solvation.It implies that in the previously mentioned range of temperature, the TBP loses the solvent moieties located in the pores.The next thermal curve after 465 °C results from the collapse of the entire framework and the breakdown of the organic ligands.So, TBP was found to exist until 465 °C; after that, a fast weight reduction of approximately 42.03% occurred, indicating the breakdown of the TBP structure.

FE-SEM
The topographical and structural morphology of the TBP was studied by FE-SEM.Figure 4 depicts FE-SEM micrographs of TBP at magnifications of ×20,000, ×40,000, ×80,000, and ×90,000.The synthesized TBP has shown a rod-shaped morphology, according to these micrographs.Additionally, the majority of particles have a width within the range of 60 to 100 nm, as shown in Fig. 5.The average width of the rods was measured to be 82 nm.

EDX and elemental mapping
The existence of carbon (C), nitrogen (N), oxygen (O), and terbium (Tb) in TBP was revealed by compositional analysis of EDX spectra, with detected weight (atomic) percents of 42.80% (63.28%), 2.17% (2.75%), 27.87% (30.93%), and 27.16% (3.03%), respectively (Figure S1). Figure S2 illustrates EDS mapping images of carbon, nitrogen, oxygen, and terbium.Both EDX spectra and mapping investigations confirmed the high-purity MOF formation by demonstrating the existence of all elements (C, N, O, and Tb) of organic ligands and metals in the final structure of the TBP with proper elemental stoichiometric proportion.

Nitrogen adsorption-desorption study
The specific surface areas of both Tb-BTC and TBP synthesized via diffusion technique were investigated using N 2 adsorption-desorption experiments at 77 K in accordance with the BET theory.The N 2 adsorption-desorption isotherms (Figure S3) of both Tb-BTC and TBP indicated a type-IV pattern characterized by an H3-type hysteresis loop, in accordance with the IUPAC classification.The incorporation of pyridine resulted in a significantly higher BET surface area of TBP (114.736m 2 g −1 ) as compared to Tb-BTC (66.572 m 2 g −1 ).

Selectivity of TBP towards DANO
Water was chosen as the solvent to examine the fluorescent sensing properties of TBP towards various FQs because TBP has good dispersion, high stability, and photoluminescent features in the aqueous medium.PL emission spectra of TBP dispersed in an aqueous medium were recorded at various excitations ranging from 323 to 383 nm (Figure S4).The maximum intensity of fluorescence emission was observed at λ ex = 353 nm.As a result, the excitation wavelength of 353 nm was chosen as the optimized excitation wavelength for further fluorescence studies to detect FQs.For selective analysis, the PL emission spectra of all suspensions (TBP/ analyte) were observed and compared (Fig. 6 and Figure S5).The outcomes showed that each of the FQs under investigation enhanced the intensity of MOF.It is important to note that although other FQs exhibited a slight to moderate enhancement effect on the fluorescence intensity of TBP, a significant enhancement was observed in the case of DANO which proved the higher selectivity of TBP towards DANO over other FQs.

Sensitivity towards DANO
TBP sensitivity towards DANO was investigated by measuring the effect of DANO on PL emission intensity at λ ex = 353 nm (Fig. 7).Remarkably, PL emission increased with increasing DANO concentration.The Stern-Volmer (SV) equation reveals a linear relationship between analyte molar concentration (C) and relative luminescent emission intensity (RI) (Qu et al. 2020).In this case, it can be properly described as RI-1 = K SV × [C], where RI represents relative intensity expressed by I/I o for enhancement and I o /I for quenching.I o and I represent the intensity values of photoluminescence emission for TBP before and after the addition of DANO, and K SV denotes the Stern-Volmer constant (M −1 ).Up to 3 ng/mL (8.40 nm), the SV plot of TBP towards DANO was observed to be linear (Fig. 8).The LOD of TBP for DANO in an aqueous medium was determined to be 0.45 ng/mL (1.25 nm) by applying the formula LOD = 3.3 σ/m, where σ is the standard deviation of the linear curve and m is the slope.

Recyclability
Recyclability is a crucial concern for sensors.So, it also looked at the PL emission characteristics of recovered TBP.As depicted in Figure S6, TBP could be reused for up to three cycles without any significant loss in its original sensing performance.Also, the PXRD pattern and FTIR spectrum of regenerated TBP after three cycles were the same as those of fresh TBP, indicating its chemical and mechanical stability (Figure S7).These results demonstrated the aqueous stability and excellent reusability of TBP for up to three cycles.

The effect of pH and response time
Using hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M) solutions, the PL intensity of the system was calculated in the pH range of 5 to 9, and it was found that the operation is pH-dependent (Fig. 9A).In solutions of acidic and basic media, it was discovered that the fluorescence intensity of DANO was lower.The highest fluorescence emission intensity was seen when DANO was added at pH 7. So, pH 7 was chosen as the optimum pH for further investigation.The luminescent enhancement may be ascribed to the deprotonation of DANO and TBP as we move from pH 5 to 7, and hence, a significant electronic interaction takes place between TBP and DANO.A further increase in pH from 7 to 9 resulted in a decrease in PL intensity which may be due to the possible precipitation of terbium hydroxide and dismantling of MOF under basic conditions (Tan et al. 2013;Gao et al. 2012).
The emission spectra of TBP-DANO suspension were monitored at intervals of 1 min up to 10 min following excitation at 353 nm to carry out the time-dependent detection of DANO. Figure 9B shows that after 1 min, the fluorescence emission attained a constant value.The fluorescence emission intensity of the suspension remained nearly unchanged throughout the contact time between DANO and TBP,

Mechanism
The interactions between MOF and analytes were studied to obtain a better knowledge of the detailed mechanism of the turn-on fluorescence enhancement behavior of TBP after the addition of FQs.Possible mechanisms of turn-on enhancement between TBP and DANO are (1) hydrogen bonding interactions (Sun et al. 2021) and (2) photo-induced electron transfer (PET) (Cong et al. 2021).
The synthesized probe contains a lot of hydroxyl and carboxyl groups, which allows it to make hydrogen bonds with DANO.Unique and effectual hydrogen bonds may be formed between the (1′s,4′s)-5′-methyl-2′,5′ diazabicyclo[2.2.1]hept-2′-yl ring, hydroxyl, carboxyl, or fluorine of DANO and the carboxyl or hydroxyl groups of TBP (Figure S8).The fluorescence intensity of TBP with DANO was significantly increased under the same excitation.The synergistic effect of hydrogen bonding can aid in the development of larger fluorophores and chromophores (Hua et al. 2018).Furthermore, it can explain the enhancement phenomenon for other FQs (LEVO, OFLO, NOR, and CIPRO) with the same mechanism as similar hydrogen interactions are present in the case of each analyte, due to which it is difficult to justify the different significant enhancement behavior of DANO.Therefore, the outcomes recommend that only hydrogen bond interactions cannot completely justify the observed enhancement behavior.Moreover, the PET mechanism may be recognized for the process of enhancement.
An effective electron transfer between the MOF and analyte is required for PET to function.As it has been proposed for many other MOFs, the PET could govern the extremely selective identification of FQs by the TBP.The turn-on enhancement resulted from the transfer of electrons from the E LUMO of analytes to the conduction band of TBP when photoexcited because the E LUMO of FQs (analytes) was higher  than the E LUMO of TBP (Figure S9) (Li et al. 2020a).The E HOMO and E LUMO energies of FQs and BTC were calculated using the Gaussian 09 package program (density-functional theory (DFT) study) by taking the basis set B3LYP/6-311G to verify the mechanism related to this turn-on enhancement (Fig. 10).The frontier molecular orbital (FMO) theory established the existence of an easy transfer of electrons between E HOMO and E LUMO of reacting species.As shown in Fig. 10, the difference between E LUMO of BTC and E LUMO of DANO was found to be lower than that of other FQs, allowing electrons to easily transfer from E LUMO of DANO to E LUMO of TBP.This agrees well with the noted highest enhancement for TBP with DANO.This implies that the electron transfer process and H-bonding interactions interact to produce the observed enhancement response.

Practicability
To investigate the practical feasibility of the developed method, experiments have been performed by spiking DANO into various environmental and biological samples such as pond water, river water, and milk samples.Decent recoveries (97.66-101.96%)have been recorded with RSD less than 2.2% (Table 1).High recovery and precise results demonstrated that the technique could be used effectively to ascertain DANO in environmental and biological samples.

Comparative study
The efficiency of the synthetic fluorescent MOF was compared against previously reported methods.As shown in Table 2, compared to other reported methods and materials, the current fluorescent probe (TBP) confirmed a better limit of detection (0.45 ng/mL) for DANO.Additionally, it exhibits good recoveries for DANO (97.66-101.96%) in biological and environmental samples with an RSD of less than 2.2%.

Conclusions
Fluorescent Tb (III)-based MOF (TBP) has been synthesized using a facile diffusion technique at room temperature for selective detection of DANO.The structural stability of the synthesized TBP was demonstrated by its ability to be recycled (i.e., up to 3 cycles).As-synthesized TBP has remarkable water stability and acts as a good fluorescent probe for the detection of DANO in an aqueous medium.
Results indicate that TBP detects DANO by producing a significant turn-on enhancement response with a LOD as low as 0.45 ng/mL (1.25 nm).The existence of hydrogen bonding and photo-induced electron transfer between the TBP and DANO can be linked to the highest turn-on enhancement.The demonstration of the sensor for FQ detection in actual

Fig. 5
Fig. 5 Particle size distribution according to FE-SEM image analysis Fig. 6 Fluorescence spectra for TBP dispersed in the aqueous solution containing different FQs when excited at 353 nm

Fig. 7
Fig. 7 Enhancement in fluorescence emission intensity of TBP upon continuous addition of DANO in the aqueous medium

Fig. 10
Fig. 10 Energy values and HOMO and LUMO plots of various FQs and organic ligand (BTC)

Table 1
Detection of DANO in milk sample, pond water, and river water by standard addition method

Table 2
Comparison of developed fluorescent probe with other reported materials and methods in the literature Developed method environmental and biological samples may fall under the purview of the expanded scope of this work.