CN111704627B

CN111704627B - Cadmium complex of 2,3,3 ', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridine, preparation method and application thereof

Info

Publication number: CN111704627B
Application number: CN202010457556.9A
Authority: CN
Inventors: 王艳宁; 王少单; 赵宇飞; 常雪萍
Original assignee: Xinyang Normal University
Current assignee: Xinyang Normal University
Priority date: 2020-05-26
Filing date: 2020-05-26
Publication date: 2022-06-28
Anticipated expiration: 2040-05-26
Also published as: CN111704627A

Abstract

The invention provides a cadmium complex of 2,3,3 ', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridyl and a preparation method and application thereof, wherein the chemical formula of the complex is [ Cd₂(bptc)(4,4′‑bpy)(H₂O)₃]·H₂O, the complex is 3-D Cd^IICP crystal structure of H₄The mixed ligand of bptc and 4,4' -bpy belongs to a triclinic system, and the space group is P ī; in the space group P ī, the asymmetric cell is composed of two Cd²⁺Ion, one bptc^4‑Ion, one 4,4' -bpy, three coordinated water molecules and one lattice water molecule. The complex can be used as a chemical sensor, can simultaneously detect AA and L-Trp through a luminescence-OFF mode and a luminescence-ON mode respectively, and has a wide application prospect in the aspect of being used as a fluorescent probe.

Description

Cadmium complex of 2,3,3 ', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridine, preparation method and application thereof

Technical Field

The invention belongs to the field of transition metal complex materials, and particularly relates to a cadmium complex of 2,3,3 ', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridyl as well as a preparation method and application thereof.

Background

Biomolecules (DNA, proteins, amino acids, biomarkers, etc.) play an important role in a variety of complex biological processes. Vitamin C (ascorbic acid (AA)) is one of the most common micronutrients for maintaining human and animal health and has been widely used in various fields of biology, nutrition, pharmacy, and chemical systems. As a drug for the treatment of poisoning, allergic reactions, atherosclerosis, scurvy and liver diseases, AA promotes the development of healthy cells and the growth of normal tissues. Studies have shown that AA deficiency may lead to scurvy and oxidative damage of lipids, while excessive intake of AA may cause gastrospasm, lithangiuria and diarrhea. Tryptophan (Trp), one of the eight essential amino acids in the human body, plays an important role in a number of physiological processes in a similar manner, such as protein biosynthesis, auxin synthesis, etc., and Trp is also a precursor of 5-hydroxytryptamine and melatonin and improves sleep, mood and mental health. Inappropriate Trp metabolism is considered to be a key indicator of certain diseases, including chronic hepatitis, pellagra, delusional disorders and parkinson's disease. Therefore, qualitative and even quantitative detection of such biomolecules is of great significance, which will promote the development process of medical diagnosis, food nutrition analysis and other technologies.

To date, a variety of biosensing methods have been explored. For example, High Performance Liquid Chromatography (HPLC), electrochemical methods, Nuclear Magnetic Resonance (NMR), atomic absorption spectroscopy have been established to determine AA. However, these methods still have some limitations. For example, electrochemical-based sensors are simple, fast, and inexpensive, but suffer from poor selectivity, unstable detection signals, and other disadvantages. Despite the large detection range and excellent selectivity of NMR and HPLC, their sensitivity is still poor. Also, at present, the most common analytical strategies for detection of Trp are based on capillary electrophoresis, gas chromatography, HPLC, UV-visible spectrophotometry and electrochemical methods, which are costly, complex to operate or poorly portable. All of these disadvantages limit their widespread application. In view of the above, there is still an urgent need to develop a cheap and simple detection method to rapidly and sensitively detect AA and Trp.

Disclosure of Invention

Based on the above, the invention aims to disclose a cadmium complex of 2,3,3 ', 4 ' -biphenyltetracarboxylic acid and 4,4 ' -bipyridyl, a preparation method and an application thereof, wherein the complex has the advantages of simplicity, rapidness and high sensitivity when being applied to detection of AA and Trp.

A cadmium complex of 2,3, 3', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridine has the chemical formula of [ Cd₂(bptc)(4,4′-bpy)(H₂O)₃]·H₂O, the complex is 3-D Cd^IICP crystal structure of H₄The mixed ligand of bptc and 4,4' -bpy belongs to a triclinic system, and the space group is P ī; in the space group P ī, the space group,the asymmetric unit consists of two Cd²⁺Ion, one bptc^4-Ion, one 4,4' -bpy, three coordinated water molecules and one lattice water molecule, two Cd²⁺The ions are Cd1 ion and Cd2 ion respectively, Cd1 and Cd2 are both positioned in an octahedral coordination center, Cd1 is surrounded by three carboxyl O atoms, one 4,4'-bpy N atom and two water molecules, Cd2 is coordinated with four carboxyl O atoms, one 4,4' -bpy N atom and one water molecule, and the Cd-N bond length is

Bond length of Cd-O is

In one embodiment, bptc is present in the complex^4-The ligand is mu₅-η¹:η¹:η¹:η²Coordination mode, wherein 2-and 3-COO-groups are respectively used as chelation mode to connect a Cd2 ion, 3 '-COO-group is a monodentate bridge combined with Cd1 ion, 4' -COO-group is combined with two separate Cd1 ions by adopting bidentate bonding mode to form non-coplanar [ (Cd1)₂(μ₂-COO)₂]A dual core unit and a distance Cd1a-Cd1g of

Fully deprotonated bptc ^4-The ligand shows the-4 oxidation state.

In one embodiment, the dihedral angle between two benzene rings in the complex is 43.58 °, and 4 COO-groups form dihedral angles of 78.31 °, 11.61 °, 50.02 ° and 44.92 ° with the corresponding benzene ring planes, respectively.

In one embodiment, the fluorescence lifetime of the complex is: tau is₁＝0.72μs，τ₂＝5.63μs。

The preparation method of the cadmium complex of 2,3,3 ', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridyl comprises the following steps: sealing 14.7mg, 0.05mmol of 2,3,3 ', 4' -biphenyltetracarboxylic dianhydride, 7.8mg, 0.05mmol of 4,4' -bipyridine and 8.0mL of distilled water in a polytetrafluoroethylene-lined stainless steel reaction kettle having a volume of 20mL, and then heating it in an oven at 120 ℃ at a constant temperature for 72 hours, wherein the distilled water is adjusted to pH 6 with HCl; after the oven was switched off, cooled to room temperature, filtered and washed with distilled water, colorless massive crystals were obtained with a yield of 48.0% based on cd (ii).

In one example, the luminescence intensity of the complex prepared by the preparation method was increased 51-fold by adding 0.5mM L-tryptophan.

In one embodiment, the quenching effect of the complex on ascorbic acid is represented by the formula: i is ₀/I＝1+ K_SV[AA]Wherein AA represents ascorbic acid, I₀And I is the luminescence intensity of the suspension of complex 1 in the absence and in the presence of AA, K_SVIs a quenching constant at a low concentration of 1.5 to 5.5. mu.M at 4.85X 10⁴M^-1The linear correlation coefficient R was calculated to be 0.9897.

In one embodiment, the enhancing effect of the complex on L-tryptophan is represented by the formula: I/I₀＝1+ K_SV[Trp]Wherein Trp represents tryptophan, I₀And I is the luminescence intensity of the suspension of complex 1 in the absence and presence of Trp, respectively, the enhancement factor K being between 0.0225 and 0.1. mu.M of the initial concentration_SVIs 9.6 multiplied by 10⁷M^-1。

The cadmium complex of 2,3,3 ', 4 ' -biphenyltetracarboxylic acid and 4,4 ' -bipyridine is used as a chemical sensor for simultaneously detecting AA and L-tryptophan through luminescence-OFF and-ON modes respectively.

The method for detecting the biological amino acid by using the cadmium complex of the 2,3,3 ', 4 ' -biphenyltetracarboxylic acid and the 4,4 ' -bipyridyl as a chemical sensor comprises the following steps: soaking 2mg of the ground complex in 2ml of deionized water, carrying out ultrasonic treatment for 30 minutes, and standing for 24 hours to form a suspension; adding 0.225M ascorbic acid, L-proline, L-valine, L-serine, L-tyrosine, L-isoleucine, L-threonine, L-phenylalanine, L-methionine, L-aspartic acid, L-leucine and L-cysteine into the suspension of the complex respectively; performing fluorescence detection at an excitation wavelength of 260nm to obtain the luminescence intensity of the 0.225M amino acid aqueous solution; the recognition capability of the complex on the biological amino acid is judged by the luminous intensity.

The selective detection method of the cadmium complex of 2,3,3 ', 4 ' -biphenyltetracarboxylic acid and 4,4 ' -bipyridyl for specific amino acids in mixed biomolecules is as follows: first, ascorbic acid, L-proline, L-valine, L-serine, L-tyrosine, L-isoleucine, L-threonine, L-phenylalanine, L-methionine, L-aspartic acid, L-leucine, and L-cysteine were added to a suspension of the complex, respectively, then, the ascorbic acid solutions were added to aqueous solutions containing 10 times the amount of the other amino acids, the other amino acids include L-proline, L-valine, L-serine, L-tyrosine, L-isoleucine, L-threonine, L-phenylalanine, L-methionine, L-aspartic acid, L-leucine and L-cysteine; fluorescence detection is carried out under the excitation of the wavelength of 260 nm; the effect of luminescence quenching of ascorbic acid on mixed biomolecules was analyzed.

In conclusion, the preparation method of the cadmium complex of 2,3,3 ', 4 ' -biphenyltetracarboxylic acid and 4,4 ' -bipyridyl is simple, the complex has the most remarkable quenching effect ON AA, the complex has the most effective enhancing effect ON L-Trp, the complex can be used as a chemical sensor, AA and L-Trp can be detected simultaneously through a luminescence-OFF mode and a luminescence-ON mode respectively, and the application prospect of the complex as a fluorescent probe is wide.

Drawings

Fig. 1 is a schematic diagram of the crystal structure of a complex 1 in the present example, in which (a) an asymmetric unit; (b) a 2-D layer; (c) two single helical chains of opposite chirality; (d) a 3-D network; (e) the topology of Complex 1.

FIG. 2 is an experimental and simulated powder X-ray diffraction (PXRD) pattern of complex 1 in this example.

FIG. 3 is an infrared spectrum of complex 1 and its ligand in this example.

FIG. 4a is H₄Fluorescence excitation spectrum and emission spectrum of bptc.

FIG. 4b is a graph showing the fluorescence excitation spectrum and the emission spectrum of complex 1.

FIG. 4c is a fluorescence lifetime map of complex 1 experimental and simulated calculations.

FIG. 5a is the luminescent response of complex 1 to different biomolecules (0.225M) in aqueous solution, with an excitation wavelength of 260 nm.

FIG. 5b is a graph of the fluorescence intensity of complex 1 of FIG. 5a for different biomolecules (0.225M) in aqueous solution.

FIG. 6a is a graph of the luminescence spectrum of an AA suspension of complex 1 as a function of AA concentration.

FIG. 6b is 1-I/I₀Linear dependence on AA concentration.

FIG. 7 is the emission spectrum (260nm wavelength excitation) of AA.

FIG. 8 is the AA selectivity of complex 1 under interference of other amino acids (5mM), wherein the fluorescence intensity is normalized to the emission intensity of 2mg of complex 1 in 2ml of pure water.

FIG. 9 is a PXRD pattern of complex 1 after soaking in different biomolecule (0.225M) aqueous solutions.

FIG. 10 is a UV-VIS absorption spectrum of different biomolecules.

FIG. 11 is a graph showing the luminescence response of complex 1 to different biomolecules (0.5mM) in aqueous solution (excitation at 260 nm).

FIG. 12a is the luminescence spectrum of a suspension of complex 1@ L-Trp with varying L-Trp concentration.

FIG. 12b is I/I₀-1 linear dependence of the plot against L-Trp concentration.

FIG. 13 shows the selectivity of complex 1 for L-Trp in aqueous solution under interference of other amino acids (5mM), and the fluorescence intensity is normalized to the emission intensity of 2mg of complex 1 in 2ml of pure water.

FIG. 14 calculated H for TD-B3LYP/6-31G₄Front track for bptc.

FIG. 15 is the emission spectra of complex 1, Trp and a suspension of complex and Trp under the same experimental conditions (excitation wavelength of 260 nm).

Detailed Description

To facilitate an understanding of the invention, the invention will now be described more fully with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

1.[Cd₂(bptc)(4,4′-bpy)(H₂O)₃]·H₂Synthesis of O (complex 1 for short):

2,3,3 ', 4' -biphenyltetracarboxylic dianhydride (14.7mg, 0.05mmol), 4,4' -bipyridine (7.8 mg, 0.05mmol) and distilled water (8.0mL) (pH 6 adjusted with HCl) were sealed in a stainless steel reaction vessel equipped with a polytetrafluoroethylene inner liner having a volume of 20mL, and then heated in an oven at 120 ℃ for 72 hours at constant temperature. After the oven power was switched off and gradual cooling to room temperature, filtration and washing with distilled water, colorless massive crystals were obtained with a yield of 48.0% based on cd (ii). C₂₆H₂₂N₂O₁₂Cd₂Theoretical value of elemental analysis of (1): c, 40.04; h, 2.86; and N,3.59 percent. Measured value: c, 40.17; h, 2.81; and N, 3.38%. Infrared (KBr, cm)^-1) The detection result is as follows: 3229(m),1696(m),1604(m),1547(s),1462(m), 1412(s),1273(m),1150(w),1074(m),861(m),817(s),765(w),1138(m),681(w), 635(w),499 (m).

2. Determining the crystal structure of the complex 1;

the calculation of the ligands was performed using the gaussian 09 program. The structure is completely optimized to the ground state at the D-B3LYP/6-31G level by the DFT method, and then the singlet state energy and the triplet state energy of the structure are calculated based on the TD-SCF method.

Data relating to Complex 1 was obtained using Mo-Ka radiation on a Bruker D8 QUEST ECO diffractometer

The obtained product is collected. The SHELXTL program was used to resolve the fit using the direct method with Olex2 as the graphical interface Structure of object 1. Using SHELXL-2014/7 at F²And refining the model. During refinement, all non-hydrogen atoms are assigned an anisotropic displacement parameter. H atoms were processed using the ridging model. CCDC number 1996924. Finally, crystallographic data of complex 1 were obtained, see table 1.

TABLE 1 crystallographic data for Complex 1

As shown in FIG. 1, X-ray single crystal diffraction showed [ Cd ]₂(bptc)(4,4′-bpy)(H₂O)₃]·H₂O is 3-DCd^IICP of H₄The mixed ligand of bptc and 4,4' -bpy belongs to a triclinic system, and the space group is P ī. In its space group P ī, its asymmetric unit is composed of two Cd²⁺Ions (Cd1, Cd2), one bptc^4-Ion, one 4,4' -bpy, three coordinated water molecules (O9, O10, O11) and one lattice water molecule (O12). Cd1 and Cd2 are both located in octahedral coordination centers, but the specific environments are different. Cd1 is surrounded by three carboxyl O atoms (O5, O7a, O8e), one 4,4'-bpyN atom (N2f) and two water molecules (O10, O11), while Cd2 coordinates four carboxyl O atoms (O1d, O2d, O3, O4), one 4,4' -bpyN atom (N1) and one water molecule (O9). The Cd-N bond length is equivalent to that of

But the bond length of Cd-O is in the range of

2,3,3 ', 4' -biphenyltetracarboxylic acid Acid (H)₄bptc) has the following structural formula:

the structural formula of 4,4 '-bipyridine (4, 4' -bpy) is as follows:

from the crystal data, it can be concluded about bptc^4-Three conclusions for the ligand: (i) bptc^4-The ligand is mu₅-η¹:η¹:η¹:η²Coordination mode, wherein four carboxyl groups show three different coordination modes: the 2-and 3-COO-groups are respectively used as chelation to connect one Cd2 ion; the 3' -COO-group is a monodentate bridge that binds the Cd1 ion. The 4' -COO-group combines two separate Cd1 ions in a bidentate bonding mode to form a non-coplanar [ (Cd1)₂(μ₂-COO)₂]A dual core unit and a distance Cd1a-Cd1g of

(ii) Fully deplasminated bptc^4-The ligand shows the-4 oxidation state; (iii) bptc^4-The molecule exhibits semi-rigid characteristics in complex 1. The dihedral angle between two benzene rings is 43.58 °, and the 4 COO-groups form dihedral angles of 78.31 °, 11.61 °, 50.02 ° and 44.92 ° with the corresponding benzene ring planes, respectively.

Complex 1 presents two single helical chains with opposite chirality. Cd1 octahedron bptc^4-The 3 '-and 4' -carboxyl groups of the ligand are connected into two helical single chains. Due to the centrosymmetric space group, the number of two helices is equal. Thus, each helical strand in complex 1 is generated by a metal-ligand interaction and can be expressed as (-Cd 1-O5-O8-). The interval between adjacent spiral chains is

Then through 4-The interaction between Cd1 and O7 of the COO-group further bridges the adjacent two helical strands to create a 1-D duplex structure. Two bptc^4-The 2-and 3-COO-bidentate groups of the ligand chelate two adjacent Cd2 ions, and the 1-D duplexes are linked together to form a 2-D layer. As shown in FIG. 1 (d), by Cd²⁺And pyridine N1, N2 atoms, the resulting 2-D layer is supported by 4,4' -bpy ligands, eventually forming a 3-D network structure. From a topological perspective, Cd2^IICan be seen as 3 connection nodes, Cd1^IICan be regarded as a 4-connection node, bptc^4-The ligand can be regarded as a 5-connection node, so that the complex structure of the complex 1 is simplified into a (3, 4, 5) -connected 3-D topological network structure with the topological symbol of (4.6)²)(4²·6⁴)(4³·6⁴·8³) As shown in fig. 1 (e).

Typical strong O-H … O hydrogen bonds are found in 3-D networks. Coordinated water molecules O10, O11 form strong hydrogen bonds with the carboxylic acid oxygen atoms O6, O7, O8, and the O … O spacing is 2.617 to

This makes the 1-D duplex more stable. Meanwhile, lattice water molecules (O12) are inserted between the layers, and form O-H … O hydrogen bonds with coordinated water O9 and carboxyl O atoms (O1, O2 and O3).

3. Powder X-ray diffraction (PXRD) analysis of Complex 1

FIG. 2 shows experimental and simulated powder X-ray diffraction (PXRD) patterns for Complex 1. The diffraction peaks of the sample of synthetic complex 1 substantially coincided with the calculated simulated pattern of the single crystal diffraction data, indicating that the synthetic sample was phase pure.

4. Infrared Spectroscopy (IR) analysis of Complex 1

Referring to FIG. 3, the spectrum is at 3000-3500cm due to the v (OH) vibration of lattice water molecules^-1Shows a broad absorption band. 1696cm^-1The strong band at (A) is characteristic of a carboxyl group. At 1700-^-1The absence of an absorption band in the region indicates complete deprotonation of the carboxyl group in complex 1, which is consistent with the results of single crystal X-ray analysis.

5. Fluorescence property study of Complex 1

As shown in FIG. 4a, free H when excited at 300nm₄The bptc ligand showed strong violet light with a maximum wavelength of 395 nm. Complex 1 showed similar maximum luminescence with only a small blue shift (15nm) observed (see fig. 4 b). It is clear that the luminescence emission band 1 can be attributed to H₄The ligands of the bptc ligands have pi-pi charge transitions within them because they have similar emission bands. The significant blue shift at 15nm is due to metal-ligand coordination interactions. Carboxylate radical and Cd^IICoordination of the ions may increase the pi x-pi transition energy gap of the carboxylate ligand, resulting in a blue shift of the emission peak.

Referring to FIG. 4c, in addition, further studies showed that the decay curve of complex 1 was fitted to a bi-exponential function and the fluorescence lifetime was calculated as: tau.₁＝0.72μs(68.37％)，τ₂＝5.63μs (31.63％)。

6. Photoluminescence sensing properties

Referring to FIG. 11, the present inventors investigated the luminescent response of complex 1 to a biomolecule. The results show that complex 1 exhibits the most significant quenching effect on AA, while complex 1 exhibits the most effective enhancing effect on L-Trp, while the other 11 biological amino acids have negligible influence on the luminescence intensity at a concentration of 0.5 mM. Thus, complex 1 can be used as a chemical sensor to simultaneously detect AA and L-Trp by luminescence-OFF and-ON modes, respectively.

7. Application of complex 1 in detection research of ascorbic acid

Referring to FIGS. 5a-5b, the luminescent response of complex 1 to some biomolecules was studied by soaking 2mg of ground sample 1 in 2ml of deionized water, sonicating for 30 minutes, and allowing to stand for 24 hours to form a suspension. 0.225M of AA, L-proline (L-Pro), L-valine (L-Val), L-serine (L-Ser), L-tyrosine (L-Tyr), L-isoleucine (L-lle), L-threonine (L-Thr), L-phenylalanine (L-Phe), L-methionine (L-Met), L-aspartic acid (L-Asp), L-leucine (L-Leu) and L-cysteine (L-Cys) were added to the suspension of Complex 1, respectively. The results show that the maximum luminescence intensity of each suspension depends to a large extent on the biomolecules. Of these, AA showed the most pronounced quenching, while the other 11 biological amino acids had negligible effect on the luminescence intensity at 0.225M concentration. This result indicates that complex 1 can act as a chemical sensor for AA by fluorescence quenching effect.

To investigate the AA sensitivity, aqueous AA solutions of different concentrations were added to the emulsion of complex 1. As shown in fig. 6a, the luminescence intensity of complex 1 gradually decreased with increasing concentration of AA. The emission intensity of the complex 1 suspension was quenched by nearly 100% by the addition of 150. mu.M AA. Furthermore, the quenching effect was analyzed by the Stern-Volmer equation: i is₀/I＝1+K_SV[AA](FIG. 6b), wherein I₀And I is the luminescence intensity of the suspension of complex 1 in the absence and in the presence of AA, K_SVIs the quenching constant, which is 4.85X 10 at low concentration (1.5 to 5.5. mu.M)⁴M^-1The calculation shows that the linear correlation coefficient (R) is 0.9897, which indicates that the compound has strong quenching effect on AA. LOD (3. sigma./Ksv) was estimated to be 5X 10 based on the Ksv value and the standard deviation of ten repeated luminescence measurements of the blank solution^-6M。

It is noted that as the concentration of AA increases, a new peak appears around 450nm, probably due to the self-luminescence of AA. To confirm this, we examined the luminescence behavior of AA and showed that the peak emission maximum was at 458nm (fig. 7) under 260nm wavelength excitation, but the peak intensity was weak and was not statistically significant and was negligible.

In fact, selective detection of specific amino acids in mixed biomolecules is essential. Therefore, we investigated a series of selective sensing and anti-interference detection by competition experiments. First, 11 different amino acids (0.225M) were added to each suspension of Complex 1, and then the AA solution was added to an aqueous solution containing 10-fold amounts of other amino acids including L-Met, L-Val, L-Leu, L-Asp, L-Ile, L-Tyr, L-Cys, L-Thr, L-Pro, L-Ser and L-Phe, respectively. Fluorescence detection was performed under excitation at a wavelength of 260 nm. As shown in FIG. 8, the luminescence quenching effect of AA did not change significantly in the mixed biomolecules, indicating that the probe for detecting AA of Complex 1 has high selectivity.

The quenching effect of AA on luminescent CPs may be attributed to the following conditions: (1) collapse of the crystal structure; (2) redox reaction between the complex and AA; (3) an anion exchange mechanism; (4) a resonant energy transfer mechanism. Here, the possible induction mechanism of luminescence quenching by AA was further analyzed.

First, PXRD patterns confirmed the structural integrity of samples of 1 in aqueous solutions of different biomolecules (0.5mM) (fig. 9). Therefore, a quenching mechanism due to collapse or decomposition of the crystal structure can be excluded. Secondly, the lack of valence-altering metal ions in the backbone of complex 1 precludes the possibility of quenching of luminescence due to redox reactions. Thirdly, ion exchange between complex 1 and AA is not possible given the neutral backbone of complex 1. Furthermore, only the aqueous AA solution had a significant overlap with the excitation spectrum of complex 1, based on the uv-vis absorption spectra of all the biomolecules we chose (fig. 10), indicating that AA can strongly absorb the energy of the excitation light and eventually lead to luminescence quenching. In addition, the quenching process of resonance energy transfer can be divided into two different quenching mechanisms (static and dynamic quenching mechanisms). In the present invention, however, the mechanism of static resonance energy transfer is the main reason for the selective detection of AA by fluorescence quenching.

8. Application of complex 1 in detection research of L-tryptophan

As shown in FIG. 11, the luminescence intensity of Complex 1 was increased 51-fold by the addition of 0.5mM L-Trp. Thus, the effect of L-Trp ON complex 1 was tested in detail to further validate the luminescence-ON detection.

Titration experiments showed that the luminescence intensity of complex 1 gradually increased with increasing concentration of L-Trp (FIG. 12 a). Quantitatively, the quenching effect can also be synthetically processed by the Stern-Volmer equation: I/I₀＝1+K_SV[Trp]And plots a good linear relationship at the initial concentration (0.0225-0.1 μ M), as shown in FIG. 12 b. Coefficient of enhancement K_SVCalculated as 9.6 in10⁷M^-1. In addition, LOD was calculated to be 63 nM. As can be seen from table 3, ligand 1 performed better in detection sensitivity and selectivity than the conventional CP-based L-Trp luminescence sensor.

TABLE 3 comparison of reported CP and L-Trp for luminescence detection.

By adding 10 times the amount of other amino acids (including L-Met, L-Val, L-Leu, L-Asp, L-Ile, L-Tyr, L-Cys, L-Thr, L-Pro, L-Ser and L-Phe to become L-Trp suspensions.) As shown in FIG. 13, there was no significant change in luminescence at 350nm prior to the addition of L-Trp, but a significant enhancement was exhibited after the addition of L-Trp, indicating that L-Trp could be detected alongside other amino acids.

In the past literature reports, the mechanism of increasing the fluorescence intensity by Trp may be the following 2 typical cases: (1) between the ligand of the compound and the singlet state of the Trp molecule

An energy transfer mechanism. The Trp singlet level is 4.3051eV, while the singlet level of most amino acids is around 5 eV. Clearly, there is a significant difference in the singlet energy levels of Trp and other amino acids, and it can be concluded that: if the sensor has an organic ligand with a singlet energy level slightly below 4.3051eV, it will have better tryptophan selectivity. To verify this conclusion, we calculated H using DFT₄Excited state of bptc ligand. The calculation shows that: h₄The singlet level of bptc is 4.48 eV. The effective excitation at 276nm (f ═ 0.1351) is comparable to the excitation observed (265nm), corresponding to the electron transition energy from its HOMO orbital to the LUMO orbital (fig. 14). Therefore, it can be excluded that

A fluorescence enhancement mechanism of energy transfer from the singlet level of Trp to the level of the 1 ligand.

TABLE 4 calculated H for TD-B3LYP/6-31G₄Absorption spectral characteristics of bptc

	Transition energy absorption wavelength oscillator strength	Transition form	Characteristics of
				S₁	4.48eV 276.45nm f＝0.1351	HOMO-＞LUMO	¹ππ^*
S₂	4.54eV 273.19nm f＝0.0670	HOMO-＞LUMO+1	¹ππ^*
				S₃	4.63eV 267.84nm f＝0.0197	HOMO-1-＞LUMO	¹ππ^*

(2) The analyte directly enhances the luminescence. I.e., the results of the selective fluorescence detection are attributed to the nature of L-Trp. As shown in FIG. 11, L-Trp can emit strong fluorescence in the range of 325-450nm, while complex 1 has only weak fluorescence in the same range, and the fluorescence intensity will automatically increase after L-Trp is added to the suspension of complex 1. However, the other amino acids did not fluoresce in the range of 325-450nm, and therefore no fluorescence enhancement was observed after the addition of the other amino acids. The emission spectra of the complex 1@ Trp solution at different concentrations also confirm this hypothesis. As shown in fig. 15, when the concentration of Trp was increased from 0.05mM to 0.5mM, the intensity of Trp increased almost the same as that of the complex 1@ Trp solution, indicating that the increase in fluorescence intensity was derived from Trp itself rather than the interaction between complex 1 and Trp.

In summary, by Cd^IIAnd semi-rigid tetracarboxylic acids H₄The bptc is constructed, and by means of the auxiliary coordination capacity of the nitrogen-containing heterocyclic ligand, the luminescent ligand 1 is successfully synthesized and applied to high-sensitivity and high-selectivity fluorescence detection of AA and L-Trp in an aqueous solution. Titration experiments and anti-interference detection indicate that complex 1 has a low LOD and high selectivity for the analyte. Further studies on the mechanism of the fluorescence probe indicate that the static resonance energy transfer and the intrinsic fluorescence characteristics of L-Trp are the main reasons for the AA fluorescence quenching and L-Trp fluorescence enhancement effects respectively. This result is believed to have positive significance in the development of light sensing technologies based on biomolecules in aqueous systems.

The above examples merely represent embodiments of the present invention, which are described in more detail and in more detail, but are not to be construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A cadmium complex of 2,3, 3', 4' -biphenyltetracarboxylic acid and 4,4' -bipyridine, characterized in that the complex has the chemical formula of [ Cd₂(bptc)(4,4′-bpy)(H₂O)₃]·H₂O, wherein H4bptc is 2,3, 3', 4' -biphenyltetracarboxylic acid, 4,4'-bpy is 4,4' -bipyridine, and the complex is 3-D Cd^IICP crystal structure of H₄The mixed ligand of bptc and 4,4' -bpy belongs to a triclinic system, and the space group is P ī; in the space group P ī, the asymmetric cell is composed of two Cd²⁺Ion, one bptc^4-Ion, one 4,4' -bpy, three coordinated water molecules and one lattice water molecule, two Cd²⁺The ions were Cd1 ion and Cd2 ion, Cd1 and Cd2 were both located in the octahedral coordination center, Cd1 was surrounded by three carboxyo atoms, one 4,4'-bpyN atom and two water molecules, Cd2 was surrounded by four carboxyo atoms, one 4,4' -bpyN atom was coordinated with one water molecule, the Cd-N bond length was 2.274(3) -2.294 (8), the Cd-O bond length was 2.228(3) a-2.487 (3) a, in this complex bptc^4-The ligand is mu₅-η¹:η¹:η¹:η²Coordination mode, in which 2-and 3-COO-groups are each chelating to link one Cd2 ion, the 3 '-COO-group is a monodentate bridge binding Cd1 ions, and the 4' -COO-group binds two separate Cd1 ions in a bidentate bonding mode to form a non-coplanar [ (Cd1) ₂(μ₂-COO)₂]A binuclear unit and the distance between Cd1a and Cd1g is 4.931(1) A; fully deprotonated bptc^4-The ligand shows an oxidation state of-4, the dihedral angle between two benzene rings in the complex is 43.58 degrees, and 4 COO-groups respectively form dihedral angles of 78.31 degrees, 11.61 degrees, 50.02 degrees and 44.92 degrees with the corresponding benzene ring planes, and the fluorescence lifetime of the complex is as follows: tau.₁＝0.72μs，τ₂＝5.63μs。

2. Use of cadmium complexes of 2,3,3 ', 4 ' -biphenyltetracarboxylic acid and 4,4 ' -bipyridine according to claim 1 for the preparation of reagents for chemical sensors, for the detection of ascorbic acid and L-tryptophan by luminescence-OFF and-ON modes, respectively.