2D Cd-MOF and its mixed-matrix membranes for luminescence sensing antibiotics in various aqueous systems and visible fingerprint identifying

Kexin M,Jing Li,Huiyn M,Yn Yng,Hu Yng,Jing Lu,Yunwu Li,?,Jinmin Dou,Sun Wng,?,Suijun Liu

a Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology,School of Chemistry and Chemical Engineering,Liaocheng University,Liaocheng 252000,China

b School of Chemistry and Chemical Engineering,Jiangxi Provincial Key Laboratory of Functional Molecular Materials Chemistry,Jiangxi University of Science and Technology,Ganzhou 341000,China

Keywords:MOF luminescence sensor Mixed-matrix membranes Real urban river water Antibiotic nitrofurazone Visible fingerprint identifying

ABSTRACT The abuse of antibiotics has brought great harm to the human living environment and health,so it is extremely significant to develop an efficient and simple method to detect trace antibiotic residues in various wastewaters.Herein,a new two-dimensional (2D) Cd-based metal-organic framework (Cd-MOF,namely LCU-111) and its mixed matrix membranes (MMMs) is sifted as luminescence sensors for efficient monitoring antibiotic nitrofurazone (NFZ) in various aqueous systems and applied as visible fingerprint identifying.The LCU-111 has good selectivity,sensibility,reproducibility and anti-interference for luminescent quenching NFZ with low detection limits (LODs) of 0.4567,0.3649 and 0.8071 ppm in aqueous solution,HEPES biological buffer,and real urban Tuhai River water,respectively.Interestingly,the luminescent test papers and MMMs allow the NFZ sensing easier and more rapid by naked eyes,only with a low LOD of 0.8117 ppm for MMMs sensor.Notably,by combining multiple experiments with density functional theory (DFT) calculations,the photo-induced electron transfer (PET) quenching mechanism is further elucidated.More importantly,potential practical applications of LCU-111 for latent fingerprint visualization provide lifelike evidences for effective identification of individuals,which can be applied in criminal investigation.

Among varied antibiotics,nitro-containing nitrofurazone (NFZ)is widely applied to treat bacterial infection of various wounds and mucosa ulcers,or as feed additive in animal husbandry and aquaculture [1,2].Its excessive intake and chronic accumulationviafood chain can induce genetic defects,immunity decline,even cancer [3–5].Due to these reasons,NFZ has been officially banned to use for humans and food related industries all over the world[6].Therefore,it is of practical significance and urgently needed to efficiently detect NFZ in various water bodies and biological systems.Compared to complicated and high-cost technologies,luminescence detection is considered as one of the most attractive method due to its cheapness,simple,convenient,real-time and high efficient virtues [7,8],which has been widely utilized to detect organic contaminants and heavy metal pollutants [9].

Among numerous luminescent sensors,MOFs with smart structures and multiple sensing sites are notably promising candidates as high-profile luminescent probes [10,11].Advantageously,by handpickedπ-conjugated organic chromophoric modules,MOF luminescent sensors can be mediated and regulated to realize the optimal spectral regions and intensities for targeted detection.So in this study,a rigid N-rich ligand 4,5-di(1H-tetrazol-5-yl)-2H-1,2,3-triazole (H3dttz) bearing largeπ-conjugated feature and multiple N-functional sites (11 N atoms in one molecule) was chosen to constructed layered 2D MOF to fulfill the NFZ detection.As is known,2D MOFs are famous star materials by exposing more accessible surface-active sites and matching improved performance.Herein,a new 2D MOF with the formula {[Cd5(dttz)2Cl4]·8H2O}n(LCU-111) was schemed and fabricated to detect antibiotics in various aqueous systems.To facilitate detection and expand its practical application,a series of MMMs based onLCU-111were further prepared using a coating method [12].The MMMs overcome the rigidity limitations of pure MOF crystals and combine the flexibility with processability of polymers to achieve a wide range of morphological features and practical sensor application devices.Besides,the homogeneous distribution of MOFs in MMMs allows for good permeability,which helps analytes to realize better contact with the MOFs compared to pure MOF sensors,resulting in improved sensing performance [13].

As expected,LCU-111can detect antibiotic NFZ in pure aqueous solution,simulated HEPES buffer and real river water by luminescent quenching with low LODs of 0.4567,0.3649 and 0.8071 ppm,respectively.More importantly,LCU-111based MMMs can also sensing NFZ with a competitive LOD of 0.8117 ppm.A possible PET quenching mechanism is validated by UV-vis,XPS,PXRD,luminescence decay lifetime assisted with DFT calculations.Interestingly,LCU-111luminescent test papers provide convenience to quickly monitor NFZ by naked eyes.Due to the strong luminescence properties ofLCU-111,a simple and visible luminescent fingerprint imaging further offers authoritative evidences to distinguish personal identity potentially toward advanced practical application [14].

TheLCU-111isolates in triclinic crystal system with space groupP-1 and shows a ladder-shaped 2D layered structure (Table S1 in Supporting information).The asymmetric unit ofLCU-111consists of two and a half Cd2+,one dttz3-ligand,two Cl-and four coordinated H2O molecules.Three Cd2+have the same six-coordinated modes with twisted octahedron geometries but different coordinated details (Fig.1a).All bond lengths and angles inLCU-111are consistent with the Cd-MOFs data reported before (Table S2 in Supporting information).Moreover,all dttz3-ligands possess the same eight-coordinated fashion to link five different Cd2+.By chelating and bridging of dttz3-and Cl-,Cd12+and Cd22+linked together to form a 1D wavy chain-like structure (Fig.1b).1D chains connected by Cd32+to generate a ladder-shaped 2D layered structure (Fig.1c).The 3D supramolecular mapping shows internal microporous structure by hydrogen bonding andπ-πstacking interaction,which can allows small guest molecules access (Fig.1d).TheLCU-111provides affluent uncoordinated N atoms and terminal Cl-as bifunctional sites in the pores,which can account for the luminescence specific recognition.

Fig.1.(a) The coordination mode of dttz3- ligand.(b) The 1D chain structure.(c)The 2D layered structure.(d) The 3D stacking structure of LCU-111.

The solid state luminescence properties of free H3dttz ligands andLCU-111were investigated at room temperature,respectively.As shown in Fig.2a,at the excitation peak of 389 nm,LCU-111appears the emission peak at 466 nm.By comparison,the free H3dttz ligands show an emission peak at 496 nm under 400 nm excitation(Fig.S1 in Supporting information).TheLCU-111emission peak occurs an obvious blue shiftvs.free H3dttz ligands.The reason is that the H3dttz ligands coordinated to Cd2+(d10) enhance their rigidity and led to easier transition of electrons.So the emission ofLCU-111are caused by n-π?orπ-π?electron transition in the ligands[9,10].Its CIE coordinates (0.1594,0.2243) are located in blue-green light emitting region (Fig.2a).

Fig.2.(a) Solid-state excitation and emission spectrum (inset: CIE coordinate of LCU-111).(b) Luminescence images of LCU-111 in different antibiotic solutions under 365 nm UV lamp.(c) The luminescence spectra and (d) intensities of LCU-111 in various antibiotic solutions.(e) The luminescence titration curves and (f) corresponding LOD of LCU-111 for NFZ in aqueous solution (inset: S-V plot).

Fig.3.(a,b) Antibiotics identification map,(c) luminescence titration experiments and (d) calculated LOD of LCU-111 in real urban Tuhai River water (inset: the urban Tuhai River acrossing Liaocheng city,China).(e) Luminescence titration experiments and (f) calculated LOD of LCU-111 in HEPES buffered solution.

LCU-111was conducted to identify antibiotics residues in various wastewaters by luminescence sensing due to its bifunctional sites.Before practical applications in various aqueous systems,the good solution stability ofLCU-111is checked using PXRD (Fig.S2 in Supporting information).The detailed sensing procedure of antibiotics in aqueous solution is offered in Supporting information.As shown in Fig.2c,the luminescence intensities ofLCU-111are different towards various antibiotics.Specifically,in NFZ solution,theLCU-111displays the lowest luminescence intensity,suggesting the strongest luminescent quenching.As shown in Fig.2d,the luminescence intensity ofLCU-111in blank aqueous solution is 4896 a.u.,while it decreases to 1.5 a.u.in NFZ solution,dropping 3264 folds.In the luminescence images taken under the 365 nm UV lamp,NFZ solution has the darkest color compared to other antibiotics (Fig.2b).This provides convenience to easily identify these antibiotics by naked eyes.The above preliminary tests indicate theLCU-111is an excellent NFZ luminescence quenching sensor in aqueous solution.

In order to investigate the optimal quenching effect ofLCU-111on NFZ,luminescence titration experiments were further carried out.With the continuous addition of NFZ,the luminescence intensity decreases obviously.When adding 210 μL NFZ,the intensity drops to only 4% compared to the value of original blank sample.From Fig.2e,theKsvofLCU-111for NFZ is calculated as 3.96×104L/mol by Stern-Volmer (S-V) equationI0/I=Ksv[M] +1,expressing the luminescent quenching efficiency [7].The LOD ofLCU-111for NFZ is 0.4567 ppm (0.1292 μmol/L) according to equation LOD=3δ/k(Fig.2f),which surpasses most reported MOF-based luminescence sensors (Table S3 in Supporting information) [10].The luminescence photos ofLCU-111in different concentrations of NFZ solutions are shown in Fig.S5 (Supporting information).As the NFZ concentration increases,the luminescence color changes gradually from bright blue-green to darker and darker.To verify the specific recognition ofLCU-111,the anti-interference experiments of NFZ by adding other antibiotics were studied and compared.As shown in Fig.S6 (Supporting information),the strength ofLCU-111remains low in the presence of various antibiotics.This shows that the recognition of NFZ byLCU-111is not affected by other antibiotics and has a good resistance to interference.Fast response is a key factor for the practical application of luminescence sensors.So a time-response experiment was also carried out,where the luminescence intensity decreases rapidly to a lowest point within 30 s after the addition of 210 μL NFZ solution,and it remains constant more than 500 s (Fig.S7 in Supporting information).In addition,LCU-111also has a good cycling stability and can be repeatedly reused after five cycles (Figs.S8 and S9 in Supporting information).

Overuse of antibiotics is harmful to humans,so the practical application ofLCU-111for the detection of NFZ in real civil wastewater from the urban Tuhai River water (located in Liaocheng city,China) and HEPES simulated biological fluid are chosen to prepare the suspensions.As shown in Figs.3a and b,Figs.S10-S12 (Supporting information),LCU-111can still recognize NFZ in real Tuhai River water samples and HEPES buffer.Compared with other antibiotics,the luminescence quenching effect to NFZ is still obvious.Similarly,luminescence titration experiments show that the luminescence intensity decreases continuously with the increasing of NFZ concentration (Figs.3c and e).TheKsvvalue ofLCU-111for NFZ is 3.38×104and 1.15×104L/mol in real river water and HEPES calculated by S-V equation (Figs.3c and e) [7],respectively.The corresponding LOD ofLCU-111for NFZ is 0.8071 and 0.3649 ppm (Figs.3d and f) [10],respectively,which is also better than most reported MOFs sensors (Table S3).The LOD in HEPES is lowest among these three aqueous systems,manifestingLCU-111is a good candidate as bio-sensor.TheLCU-111still has strong antiinterference capacity for identifying NFZ against other antibiotics in real Tuhai River water (Fig.S10 in Supporting information).LCU-111also has a good cycling stability after five cycles (Fig.S13 in Supporting information) and fast response time (Fig.S14 in Supporting information) in real Tuhai River water.It is worth noting that the real Tuhai River water samples contain lots of complex substances and undulated pH values as a flowing river acrossing the Liaocheng city,however theLCU-111still has stable recognition performance in real river water,suggesting a promising application in practical wastewater detection.

Due to above excellent sensing capability ofLCU-111toward NFZ in various aqueous systems,the luminescent MMMs were further made by drawdown coating process so as to expand its practical application [12].Four types of MMMs are prepared (Fig.S15a in Supporting information) and the mass ratios ofLCU-111are 0 wt%,2 wt%,4 wt% and 8 wt%,respectively.Taken 8 wt% MMMs as example,the PXRD spectra before and after sensing NFZ are in agreement with the crystalLCU-111,provingLCU-111based MMMs have good crystal structure stability (Figs.S17 and S18 in Supporting information).The selected 0 wt% and 8 wt% MMMs both have good ductility and plasticity (Figs.S15b-e in Supporting information).Images of 2 wt% and 4 wt% MMMs are also taken under natural light and 365 nm UV lamps (Fig.S19 in Supporting information).The SEM of 0 wt% and 8 wt% MMMs both show smooth and trim surfaces (Figs.S15f and g in Supporting information).The difference is that the 8 wt% MMMs displays obvious porous structure.According to previous reports,MMMs have better sensing performance when the MOF content is 8 wt% [12].From Fig.S20 (Supporting information),the luminescence intensity of 8 wt% MMMs is four times higher than that of pure MMMs.Also,the luminescence sensing intensitiesversustime of four MMMs in NFZ solution were monitored,which still show 8 wt% MMMs are stable (Fig.S21 in Supporting information).So subsequent experiments were carried out using 8 wt% MMMs.

As shown in Figs.4a and b,8 wt% MMMs also show excellent luminescence sensing toward NFZ as expected.The luminescence intensity in blank aqueous solution is 13 times lower than in aqueous solution.As can be clearly seen from illustration of Fig.4a,MMMs become darken in NFZ solution compared to other antibiotic solutions under UV lamp (Fig.S22 in Supporting information).Moreover,the quenching titration experiment is shown that the luminescence intensity decreases gradually with the addition of NFZ solution (Fig.4c).After the addition of 140 μL NFZ,the luminescence intensity reduces to 8%versusthe original value.The calculatedKsvofLCU-111MMMs for NFZ is as high as 6.23×104L/mol[7],and the corresponding LOD is 0.8117 ppm (Figs.4c and d),also lower than most reported MOFs sensors (Table S3) [10].Competition experiments show thatLCU-111MMMs still have strong antiinterference ability for quenching NFZ in mixture with other antibiotics (Fig.S23 in Supporting information).TheLCU-111MMMs also have fast response time for only 20 s,and then remain stable for more than 300 s (Fig.S24 in Supporting information).These results indicate thatLCU-111MMMs have efficient and stable sensing performance of NFZ in aqueous solution,illustrating its well practical application value.

Fig.4.(a) The luminescence spectra (inset: MMMs before and after immersion in NFZ) and (b) the luminescence intensities of LCU-111 MMMs in various antibiotic solutions.(c) The luminescence titration experiments and (d) the calculated LOD of LCU-111 MMMs.

To facilitate the monitoring more simply in real-world application,luminescent test papers ofLCU-111were made to quickly detect NFZ (Fig.5a) and other antibiotics (Fig.S25 in Supporting information).When gradually dropped NFZ solution onto the test papers,their colors are turning ever more deepened under UV light,that is,from light blue to dark blue (Fig.5a).Moreover,fingerprint is one of the most concerned material evidences to identity recognition due to its uniqueness of everyone and unchanged throughout life.So based on the strong luminescence properties ofLCU-111,a simple and visible luminescent fingerprint identification was proposed.Fingerprints were printed on smooth surfaces of different substrates such as plastic,steel,glass and various papers [14].The fingerprints are invisible in sunlight,but vivid fingerprint patterns on a variety of substrates appear under 365 nm UV light(Figs.5b-h).Interestingly,the fingerprint patterns are clearer on steel and glass than on plastic,print paper and invoice paper,respectively,indicating that theLCU-111particles bind more strongly on steel and glass surfaces than on others.Furthermore,threelevel detailed fingerprint features can be clearly observed by naked eyes,such as patterns of whorls and loops (level-one),ridge ending and bifurcation (level-two),and even scars (level-three) (Figs.5b-f).These unique characteristic informations provide authoritative evidences to distinguish personal identity toward advanced practical application ofLCU-111.

Fig.5.(a) Luminescent test papers of LCU-111 for NFZ under UV lamp.Luminescence fingerprint photographs of LCU-111 on (b) steel,(c) glass,(f) plastic,(g) print paper,and (h) invoice paper.(d,e) The enlarged bifurcation and whorls of the fingerprints on glass.

The quenching mechanism ofLCU-111for NFZ was further verified by XPS,PXRD,UV-vis,luminescence decay lifetime and DFT calculations.Firstly,the PXRD after 3 days immersed in NFZ solution is the same as original crystalLCU-111,indicating the framework structure ofLCU-111remains intact after sensing (Fig.S2),ruling out the structural collapse led to luminescent quenching.Secondly,the UV-vis absorption spectrum of NFZ-treatedLCU-111does no overlap with the luminescence excitation spectrum(λex=389 nm) of the originalLCU-111(Fig.S26 in Supporting information),suggesting that the competitive energy absorption of the NFZ andLCU-111is not responsible for the luminescence quenching.Thirdly,the XPS show that the binding energies of Cd 3d,O 1s,N 1s and Cl 2p all arise blue-shift from 406.1,532.9,400.6,198.8 eV to 406,532.8,400.3,198.7 eV,respectively,indicating the occurrence of interaction between NFZ and theLCU-111skeleton (Figs.S27 and S29a-d in Supporting information).Lastly,the interaction is further verified by the luminescence decay lifetime results (Fig.S28 in Supporting information),which reducesca.12% for NFZ-treatedLCU-111(3.6515 μs)vs.the original lifetime(4.1278 μs).

Based on the above series of experiments,the luminescence quenching ofLCU-111toward NFZ may possibly be attributed to the PET mechanism,where electron transfer can cause weak interaction,and further result in luminescence quenching.A typical PET system is composed of a receptor,a spacer,and a fluorophore donor.In a PET luminescent molecular probe,the conversion between the recognition group and fluorophore is accomplished by light-induced electron transfer.Generally,the PET process is triggered by the energy gap of the LUMO between the photo-excitation donor and acceptor [15–18].Since the luminescence ofLCU-111is mainly derived from the H3dttz ligands with rich delocalized electrons,and the NFZ has a strong electronabsorbing group (-NO2),the PET process can occur from H3dttz to the LUMO of NFZ [18].To further verify this conjecture,DFT calculations were performed using the B3LYP/6-31G?motif [15–18].As shown in Fig.S29e (Supporting information),the LUMO energy level of H3dttz (-2.00 eV) is higher than that of NFZ (-3.02 eV),so the PET execute electron transfer from H3dttz to NFZ,and thus induce the best quenching performance.Moreover,in contrast,only the HOMO and LUMO energy of the NFZ among all sensing antibiotics both locate between the HOMO and LUMO energy of H3dttz(Fig.S30 in Supporting information),which further validates more easily recognized of NFZ.Therefore,the luminescence quenching mechanism ofLCU-111toward NFZ can be attributed to the PET mechanism.

In conclusion,a new 2D Cd-MOFLCU-111has been picked as a luminescent quenching sensor for efficient detecting antibiotic NFZ in aqueous solution,HEPES biological buffer,and real urban river water with low LODs of 0.4567,0.3649 and 0.8071 ppm,respectively.The sensor is gifted with good selectivity,sensitivity,stability and anti-interference ability in above various aqueous systems due to more exposed functional sites of 2D material.Moreover,the luminescent test papers ofLCU-111realize quickly,real-time and visual monitor NFZ by naked eyes.Notably,LCU-111based MMMs further provide flexible practical applications for quenching NFZ also with a low LOD of 0.8117 ppm.A reasonable PET quenching mechanism is demonstrated by experiment metrics plus DFT calculations.Latent fingerprint visualization ofLCU-111can support the security sectors to precisely identify criminals.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos.21771095 and 22061019),the Natural Science Foundation of Shandong Province (Nos.ZR2021MB114 and ZR2021MB073),and the Youth Innovation Team of Shandong Colleges and Universities (No.2019KJC027).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.cclet.2023.108227.