Zinc(II) Carboxylate Coordination Polymers with Versatile Applications

This review considers the applications of Zn(II) carboxylate-based coordination polymers (Zn-CBCPs), such as sensors, catalysts, species with potential in infections and cancers treatment, as well as storage and drug-carrier materials. The nature of organic luminophores, especially both the rigid carboxylate and the ancillary N-donor bridging ligand, together with the alignment in Zn-CBCPs and their intermolecular interaction modulate the luminescence properties and allow the sensing of a variety of inorganic and organic pollutants. The ability of Zn(II) to act as a good Lewis acid allowed the involvement of Zn-CBCPs either in dye elimination from wastewater through photocatalysis or in pathogenic microorganism or tumor inhibition. In addition, the pores developed inside of the network provided the possibility for some species to store gaseous or liquid molecules, as well as to deliver some drugs for improved treatment.


Introduction
When we focus on properties, Zn(II) complexes seem unattractive considering both their diamagnetism and lack of color. However, when we evaluate the relationship with living organisms, it can easily be noticed that zinc is the second most abundant transition ion in the series of essential elements, indispensable for a huge number of biological processes. This trace element is critical for cell growth, genomic stability, signal transcription factors, and for the structure and function of a wide range of cellular proteins [1][2][3][4][5].
In thousands of proteins, zinc participates in enzymatic catalysis, structural organization, and/or function regulation. Characteristics such as fast ligand exchange, stereochemical flexibility, and redox inertness sustain its selection in the function of numerous proteins [6]. For these biomolecules, Zn(II) acts mainly as a Lewis acid for enzymes involved in hydrolytic process regulation, as well as a structural factor in both nucleic acid synthesis and their activity control [1][2][3][4][5][6].
Based on the good Lewis acid activity observed in living systems, this ion has found applications in several catalytic processes [7] and in developing species with biological activity [5,8,9]. Moreover, its structural control through coordination generates valuable applications as sensors based on luminescent properties for species with proper selected ligands [10][11][12]. Its stereochemical versatility also allows a structural arrangement in the complex's network that generates pores and, thus, induces the ability of some materials to storage gaseous, liquid or solid species [13][14][15][16][17][18][19].
Some of these properties are linked to a polymeric structure of complexes that can be achieved by using a proper molar ratio and one or more ligands that can act as a bridge. The ability of monocarboxylate derivatives to link Zn(II) ions into coordination polymers (CPs) by forming two-or three-atom bridges is well established (Figure 1) [20].   Moreover, this ability is enhanced for species containing two or more such groups in their structure. For these species, besides the most common two-atom bridge, there are also bridges in the network structure connecting between three to eight atoms. Hence, several aliphatic and especially aromatic di-, tri-or tetracarboxylate derivatives have been employed for the synthesis of Zn(II)-CPs with diverse structures and various properties. A selection of coordination modes adopted by carboxylate groups in Zn(II) carboxylatebased coordination polymers (CBCPs) is depicted in Figure 1.
Among these materials, metal-organic frameworks (MOFs) represent a special class that provide the advantage of both ultrahigh porosity and specific surface area, high thermal stability, adaptable surface chemistry, adjustable crystal structure and morphology, open metal sites and robustness [18]. The current limitations of Zn-based MOFs in comparison with others Zn-CBCPs come from the fact that is difficult to control the morphology and structure [18] and to create a mixture either of organic linkers, carboxylate and N-donor ligand or of cations Zn(II) and another cation [21].
On the other hand, Zn(II) is suitable for developing such CPs since its d 10 configuration provides flexible stereochemistry and, consequently, the geometries of complexes can be easily modulated from a tetrahedral geometry through square pyramidal and trigonal bipyramidal geometries to an octahedral geometry, with different distortion degrees, stereochemistry achieved by a proper selection of ligands and molar ratio, respectively.
Furthermore, due to the lability of bonds associated with zero-field stabilization energy in such complexes, the coordinative bond formation is reversible and, thus, enables ligand rearrangement during the polymerization in order to provide diverse ordered networks [20,22]. Consequently, Zn(II) complexes can easily adopt a wide range of 1D, 2D and 3D arrangements, but these also depend on molar ratio, reaction conditions, ancillary ligand nature and coordinative abilities.
A literature survey evidenced several carboxylate derivatives, especially rigid ones such as di-and tricarboxylate benzene derivatives, that are able to develop polymeric species by coordination to Zn(II). Their structure can then be modulated either through selfassembly by carboxylate alone or by using another polydentate species acting as a bridge. These ancillary ligands are usually selected by heterocyclic nitrogen-based species that are also rigid, such as pyridine, imidazole, triazole, bipyridine or phenanthroline derivatives, which, in addition to modulating the topology and porosity, can provide the species with a fluorophore component.
In addition to design interesting structures, the considerable interest in this field is directed towards obtaining CBCPs with useful properties, such as luminescence, porosity, and catalytic, antitumor and antimicrobial abilities, that lead to potential modern applications in several fields, such as sensors, photocatalysts, drugs, or storage materials (Scheme 1).
The most recent data concerning these applications are summarized by categories in the following sections. The most recent data concerning these applications are summarized by categories in the following sections.
To fulfill the objectives set for this paper, a literature survey was performed using the most important and commonly accessed databases such as Scopus, Science Direct, and Web of Science. For this approach, keywords according to the paper goal were selected, with zinc coordination polymer being the first used. Afterwards, supplementary keywords such as carboxylate ligand, luminescence, sensor, storage, catalyst, biological activity, and MOF were included in the search.
Based on this procedure, relevant high-quality scientific papers, reviews, and reports were collected, which were then sorted in an organized fashion using the criteria date of publishing, properties and applications (sensors, catalysts, biological activity, storage materials, miscellaneous, etc.) of identified complexes, and carboxylate coordination modes, and, afterwards, subjected to data extraction.
Moreover, some relevant papers identified in accessed databases were used for the backward and forward snowballing technique to identify additional ones which were included in our database and further used for data collection.
Concluding, the data collection resulted from a combination of database search and snowballing. Searches were limited to papers recently published, mainly starting with the year 2012.

Zinc(II) Carboxylate Coordination Polymers Developed for Different Applications
The compounds reviewed in the following sections show luminescent properties or have robust and thermally stable open-framework structures giving rise to different porosity, which is a valuable characteristic for sorption or selective inclusion of small guest molecules. The photocatalytic abilities were directed to the elimination of drugs or dyes from wastewater, while the biological activity was targeted on pathogenic microorganism or tumor inhibition with the purpose of developing new drugs. Additional applications include the use of Zn(II)-CBCPs as the anode material for lithium-ion batteries (LIBs), in the area of 3D printing or to obtain optoelectronic devices. Further, some species could be used to synthesize nanoparticles with various properties.

Zinc(II) Carboxylate Coordination Polymers with Luminescent Properties
Among water pollutants categories, nitro derivatives, volatile organic compounds, dyes, antibiotics and ionic inorganic species are of major concern because they are noxious for both human health and the environment. Many organic nitro compounds and organic solvents used in industrial activities are of great concern because they could be accidentally discharged into wastewater, leading to environmental unwanted issues. Hence, their detection and sensing require immediate from chemists, this being an aspect that needs to be solved. Among the available methods, fluorescent sensors, characterized by a high sensitivity, fast response, and user-friendly operation, have attracted extensive research interest in recent years [12].
Luminescence of Zn(II)-CBCPs arises from the characteristics of the supplementary organic fluorescent ligands, such as an aromatic conjugated system, while small guest molecules can either enhance or quench this phenomenon. The origin of the luminescence in such species can be related to one or several electron transfer mechanisms, such as intraligand (IL), ligand-to-ligand (LLCT), and metal-to-ligand (MLCT) charge transfers [10,11].
If the carboxylate linker displays extended π systems, this will consequently be the source of luminescence. If not, another linker with such characteristics can be engaged in CP synthesis. Moreover, both ligands could have an electron rich π cloud and can, therefore, contribute to luminescence.
Since CPs of Zn(II) have the ability to regulate the emission wavelength of organic materials, several species based on benzene di-or tricarboxylate rigid ligands as tectons were developed as luminescent materials. For some complexes, only the ability to exhibit luminescence was reported, but many of them have proved the ability to act as sensors, either for inorganic or organic species, especially for pollutants originating from industrial and hospital wastes.
All these species are promising luminescent sensors in the detection of various analytes in water and could, as a result, be developed as hybrid inorganic-organic photoactive materials.
The nanospecies [Zn(cpma)Cl 2 ] n (21) (cpma = 9,10-bis((4-carboxylatopyridinium-1methylene)anthracene) was recently reported as a probe for live-cell imaging studies. The complex retained the viability of the human colorectal adenocarcinoma cell line (HCT-15) even at the highest concentration of 50 µM. Moreover, the fluorescent images showed that the CP was localized into the cell cytoplasm instead of the nucleus [55].
The luminescent CPs [Zn 2 (3-bpat) 2 -H 2 hip = 5-hydroxyisophthalic acid, 5-H 2 mip = 5-methylisophthalic acid) were prepared as optical sensors to detect target analytes. All species showed a good fluorescence quenching response to Fe(III) and MnO 4 − with high selectivity and sensitivity, the DLs for both analytes being in the nanomolar range. Moreover, cation and anion competition experiments indicated that the luminescence quenching response of both analytes were almost unaffected by interfering ions [57].
The CP {[Zn(1,2,4-btc)(Hdpa)]·H 2 O} n (66) (Hdpa = 4,4 -dipyridylamine) was synthesized under pH-controlled hydrothermal conditions. This complex exhibits excellent luminescence in both the solid state and in solution and can act as a multi-responsive luminescent sensor for Fe(III) and MnO 4 − with high selectivity and sensitivity [85]. As a result, the proper selection of multifunctional organic ligands is very important for the preparation of Zn(II)-CBCPs, because it is crucial for obtaining structural diversity and good luminescent properties. Among numerous ligands, polycarboxylate, especially derived from rigid aromatic systems that exhibit both a large π-conjugated skeleton and the ability to coordinate as a bridge, are excellent candidates to construct such functional species. The properties can be further tuned by selecting a second ligand with the same characteristics, such as pyridine, imidazole, or triazole derivatives.
The proper selection of organic fluorophores (carboxylate and N-donor ligand) as rich aromatic moieties generally led to Zn-CBCPs with a broad range of emission energies. This aspect, together with pore dimensions, allow the specific recognition either of an inorganic or organic species, that can, in addition lead, to the possibility of developing sensors useful for a large variety of pollutant monitoring processes.

Zinc(II) Carboxylate Coordination Polymers with Catalytic Properties
Zinc is a metal with good catalytic performance based on its Lewis acid ability. Therefore, in the literature, many Zn(II) complexes have been reported with various types of ligands which are used for their catalytic properties. For instance, Zn(II)-CBCPs may catalyze the degradation of different organic dyes or influence the evolution of certain chemical reactions, as is presented in detail below.
Organic dyes and pharmaceuticals are identified frequently in water and wastewater, they resist biodegradation and present hazardous environmental effects. Among the reported technologies to remove these pollutants from waters, the use of photocatalytic degradation is a promising route to break down organic molecules into nontoxic species [132,133]. [86] as efficient photocatalysts for the degradation of water pollutants, mainly of ibuprofen (IBP) whose presence is often identified in water and in wastewater treatment plant effluents. Over time, IBP degradation was investigated through different methods but the identification of the optimum method is still under study. The above-mentioned complexes were studied as heterogeneous photocatalysts in different conditions. Studies revealed that the photocatalytic degradation of IBP is more efficient in the presence of (67) under UV irradiation, whereas in the case of both complexes, the addition of H 2 O 2 significantly decreased IBP levels.
An interesting and useful reaction in organic chemistry is Knoevenagel condensation, a method used to generate new carbon-carbon bonds which occurs with nitrogen-based catalysts [94]. The challenge for this reaction is to find the most efficient catalyst; hence, CP [Zn(paph)(NMeF)] n ·n(NMeF) (78) (H 2 paph = 5-{(pyren-4-ylmethyl)amino}isophthalic acid, NMeF = N-methylformamide) was under evaluation from this point of view in the Knoevenagel condensation of benzaldehyde and malononitrile in supercritical CO 2 (scCO 2 ) medium [95]. The evaluation of (78) as a catalyst in scCO 2 was conducted using different co-solvents and the reaction yield increased from aprotic to protic co-solvents, the full conversion being recorded in the case of water.
Another exploited application of CPs is the possibility to act as catalysts in the electroreduction of CO 2 . This reaction consists of the conversion of CO 2 to more reduced species, this being a promising approach to obtain fuels or other chemicals, or to reduce anthropogenic CO 2 emissions [135]. In light of this information, complex [Zn 2 (daba) 4 (4,4 -bipy)] n (79) (Hdaba = acid 4-diallylamino-benzoic) has proven a high efficiency and selectivity as an electrocatalyst for CO 2 reduction to methanol, formaldehyde, and formic acid, which were identified by means of 13 C NMR spectral data [96].
Having in view that CPs are used as electrocatalysts in the hydrogen evolution reaction (HER) due to their high surface area and active centers, some researchers [97,98] have synthesized the CP [Zn 2 (tzpi)(OH)(H 2 O) 2 ] n ·2nH 2 O (80) (Figure 3b) using 5-(4-(tetrazol-5yl)phenyl)isophthalic acid (H 3 tzpi), in order to investigate electrocatalytic performances. Consequently, they doped Co(II) ions into the CP framework to generate active sites so that the resulting product has a high specific surface area and presents excellent electrocatalytic properties for HER. Mohammadikish and co-workers [136] synthesized a bi-metallic CP with a Schiff base ligand resulting from the condensation of a functionalized aldehyde and p-aminobenzoic acid, coded Zn-Mo-ICP (81) (ICP = infinite coordination polymer). The complex has proven its ability to adsorb MB (100%) and MO (52%), suggesting the tendency to more efficiently adsorb cationic dyes (MB). Moreover, complex (81) presents catalytic activity in the epoxidation of olefins with TBHP (t-butylhydroperoxide) as an oxidant, presumably also due to the existence of an N,O-donor Schiff base ligand. Studies have proved that the mentioned CP could be recycled four times without the loss of activity.
The exploitation of the Lewis acid character of Zn(II) afforded the development of several valuable materials with catalytic behavior. This ability was modulated through a large variety of carboxylate and N-donor ligands, the assembly of which generated pores able to specifically recognize organic species (dyes, drugs, etc.) as substrates. Coordination at Zn(II) centers led to substrate activation and, finally, their degradation.

Zinc(II) Carboxylate Coordination Polymers with Biological Properties
Nowadays, bacterial infections and multi-drug resistance have become a matter of global concern, with many complexes being screened in order to overcome this issue [137]. Among the evaluated complexes, Zn(II) containing ones are taken into consideration due to the biocidal effect of this ion [138].
Among CPs, [Zn 1.5 (CH 3 COO) 2 (4,4 -bipy) 2 ] n (ClO 4 ) n ·nH 2 O (82) has proven its biocidal activity against Gram-negative (Escherichia coli) and Gram-positive bacteria (Staphylococcus epidermidis) in both liquid and solid growth media compared with zinc sources, this behavior being related with a gradual and localized release of Zn(II) ions. Assessment of minimal inhibition concentrations (MIC) for compound (82) against E.coli and S. epidermidis led to values of 6.1 ppm and 4.6 ppm, respectively. The biocidal mechanism was associated with reactive oxygen species (ROS) generation and membrane disruption [99].
A series of CPs with indolecarboxylic acids were synthesized and subjected to biological tests [100]. Thus, complexes [Zn(I3aah  2 ] n (87) (H5-MeOI2cah 2 = 5methoxyindole-2-carboxylic acid) were tested for antibacterial activity against Grampositive (Bacillus subtilis, Lysteria monocytogenes, Staphylococcus aureus) and Gram-negative (E. coli, Pseudomonas aeruginosa) bacterial strains, and for antifungal activity against Aspergillus niger and Candida albicans. The results indicated that all complexes are more active than corresponding free ligands and present good activity against B. subtilis. Complex (85) is the most active against this strain (with an inhibition zone above 25 mm), followed by (86) (with an inhibition zone above 20 mm). Related to the activity against L. monocytogenes, complexes (84) and (86) exhibited inhibition zones above 20 and above 10 mm, respectively.
Concerning the antifungal activity, the tests evidenced that from all tested complexes, only (86) presents activity against A. niger. lus subtilis, Lysteria monocytogenes, Staphylococcus aureus) and Gram-negative (E. coli, Pseudomonas aeruginosa) bacterial strains, and for antifungal activity against Aspergillus niger and Candida albicans. The results indicated that all complexes are more active than corresponding free ligands and present good activity against B. subtilis. Complex (85) is the most active against this strain (with an inhibition zone above 25 mm), followed by (86) (with an inhibition zone above 20 mm). Related to the activity against L. monocytogenes, complexes (84) and (86) exhibited inhibition zones above 20 and above 10 mm, respectively. Concerning the antifungal activity, the tests evidenced that from all tested complexes, only (86) presents activity against A. niger.  For [Zn(bfmta)(H 2 O) 2 ] n (88) (H 2 bfmta = 2,5-bis(furan-2-ylmethylcarbamoyl)terephthalic acid), biological tests were developed using Gram-positive (B. subtilis, S. aureus), Gramnegative (E. coli, Salmonella typhi) bacterial and fungal strains (Penicillium expansum, Botrydepladia thiobromine, Nigrospora sp., Trichothesium sp.) and were discussed in comparison with ciprofloxacin. The results evidenced that the antibacterial and antifungal activity of the complex is lower than of the standard drug but higher than of the free ligand (bfmta). The inhibition zones against bacterial strains are above 30 mm, whereas they are between 24 and 30 mm against fungal strains. Additionally, the antibacterial activity of (88) is higher than that of similar complexes with Mn(II), Co(II), or Ni(II) [101].
A different Zn(II)-CP, [Zn 4 (bdc) 4 (ppmh) 2 (H 2 O)] n (89) (H 2 bdc = 1,4-benzene dicarboxylic acid; ppmh = N-pyridin-2-yl-N'-pyridin-4-ylmethylene-hydrazine), has proven its efficiency as an antibacterial agent against E. coli and S. aureus, being more active than the free ligands. As liver cancer is a malignant tumor with high incidence worldwide, finding new therapeutic agents has gained great importance. For example, complex (89) is also a potential antitumor agent against liver (HepG2) cell lines. The assay indicates that viability decrease in parabolic dependence with increasing concentration with an LD 50 value of 42.2 ± 2.3 µg/mL. Ligands did not evidence any impact at levels up to 120 µg/mL and the positive control (cisplatin) had an LD 50 value of 12.6 ± 2.8 µg/mL [102].
Five Zn(II) CPs with 5-azidoisophthalic acid (N 3 -H 2 ipa) and various N-donor coligands were synthesized by  and tested for their cytotoxic effects on human colorectal carcinoma cell lines (HCT 116). Among these, [{Zn(H 2 O) 0.5 (N 3ipa)(phen)}] n (95) has proven to be the most active with IC 30 , IC 50 and IC 70 values of 0.57, 25.56 and 50.55 µg/mL, respectively, which are much lower in comparison with other CPs.
Treatment of glomerulus nephritis was efficient by the application of (58), this being evaluated by a reduction of accumulation of ROS in glomerular epithelial cells. The level of ROS decreased significantly after treatment with (58) in comparison to after treatment with the positive drug Nifedipine [78].  reported that [Zn(tptc) 0.5 (2,2 -bipy)(H 2 O)] n (99) (H 4 tptc = p-terphenyl-2,2 ,5 ,5 -tetracarboxylic acid) was able to reduce the weight and length of thrombus in animal models, the inhibitory effect being directly related with the dose.
Since Zn(II) ion is void of redox activity, the biologic behavior of compounds is also connected to its Lewis acid ability, enhanced by coordination to proper ligands. Furthermore, its stereochemical versatility allows the coordinative interaction with biomolecules (DNA, enzymes, etc.), followed by their splitting through hydrolytic processes.

Zinc(II) Carboxylate Coordination Polymers as Storage Materials
Besides interesting structural topologies, the design of CPs with a porous morphology has gained interest due to their ability to store different gaseous products, this being a manner of adsorbing inclusively greenhouse gases.
Although such complexes are compared to zeolites, the advantage over zeolites is the possibility to alter the architecture and functionalize the pores [139]. Storage properties are related with a high porosity and a high specific surface area. Additionally, this type of CP is able to trap organic molecules, including solvents [120], which is useful for separation purposes.
In such CPs, N-donor ligands combined with polycarboxylates as anionic linkers are used as a necessity to construct porous structures able to adsorb gases, which is illustrated in this section.
CP [Zn 2 (N 3 -ipa)(4,4 -bipy)(DMF) 1.5 ] n (109) (N 3 -H 2 ipa = 5-azidoisophthalic acid) was tested for sorption properties of different gases (N 2 , O 2 , CO, CO 2 , C 2 H 2 ) [118]. After drying at 120 • C in vacuum for 6 h, the compound resulted in being non-porous for N 2 , O 2 , and CO at 77 K. At higher temperatures, it was found that (109) was able to adsorb N 2 , O 2 , and CO (at 120 K), and CO 2 (110) which presents high adsorption selectivity towards CO 2 and the potential to be a good adsorbent material at 77 K. Additionally, it was found that the compound exhibits a higher adsorption selectivity for CO 2 /CH 4 mixture against certain zeolites.
Besides the gas storage properties, CPs may present selective sorption properties of different organic compounds. Accordingly, Zaguzin and co-workers [121,122]  for which the selectivity of sorption of different organic substrates from gas phases was tested. The results showed that (112) retained 1,2-dichloroethane from its mixture with benzene more readily in comparison with (113) and (114). Furthermore, (112) presented the best selectivity for benzene/cyclohexane mixtures, followed by (113) and (114).
Considering their mesoporous structures, these MOFs were tested for their potential to deliver IBP in phosphate-buffered saline solution by impregnating them with 30, 50 and 80 wt% IBP. The effect of IBP concentration on the release rate was evaluated and the maximum effective loading for each compound was estimated. The results evidenced that IBP loading was more effective in (115) in comparison with (116). More specifically, (115) could deliver a higher IBP concentration than (116) (50 vs. 30 wt%, respectively) at a faster rate. This study demonstrated that compound (115) is suitable for efficient drug delivery and dissolves in the process of drug release, this being an advantage for the drug delivery process [124].
These Zn(II)-based MOFs are studied as drug delivery systems due to their excellent biocompatibility, easy functionalization, high storage capacity, and excellent biodegradability [21]. In addition, considering that Zn-MOFs are developed based on a biocation, these are non-toxic species.
The modulation of porous cavities through carboxylate and ancillary ligands, the molar ratio and the reaction conditions allowed the development of a variety of materials able to selectively absorb and store certain organic and inorganic gaseous species, as well as carry drugs for a more efficient administration.

Zinc(II) Carboxylate Coordination Polymers for Miscellaneous Applications
In this section are gathered other applications of Zn(II)-CBCPs identified in the literature data besides the aforementioned ones.
For instance, such species can be used as anode material for lithium-ion batteries (LIBs). Taking into consideration that LIBs are used for various portable devices and electrical vehicles even in military purposes, this application gains a lot of importance.
The compound PDMS-COO-Zn (PDMS = poly(dimethylsiloxane)) was reported for several applications. The tests evidenced that materials made from polymer present soft and viscoelastic properties at high temperatures, and due to rapid softening and hardening properties, it could be successfully used in medical fields (orthopaedic immobilization and external fixation systems). Another application of this species is in 3D printing, because when it is heated at 120 • C, it turns into a viscous liquid which quickly forms a rigid solid upon cooling. Additionally, by doping the compound with conductive materials, the resulting species are suitable for preparing conductive composites [141].
One-dimensional CP {[Zn(npdi)(1,3-bdc)]·H 2 O} n (122) (npdi = 1,1 -(4-nitro-1,3-phenylene)bis(1H-benzo[d]imidazole) was used to synthesize multifunctional nanoparticles (NPs) (Ag, Au/Au 2 O 3 , Pd, Cr/Cr 2 O 3 /CrO 2 , Cu/Cu 2 O, Fe/FeO) at room temperature in the absence of a reducing agent [141]. The ability of this CP to be used in such a way arises from the ability of carboxylate at the inner surface of the cavities to anchor the metal precursors. The resulting NPs present various properties: Ag NPs present antibacterial properties; Ag, Au, and Cu NPs present ferromagnetic properties within the framework at room temperature. The same authors reported that compound (122) is able to sequester Cr(VI) toxic species into Cr NPs nontoxic ones [130].
Moreover, compound (89) was reported for Schottky diode barrier properties. Its conductivity is higher by one order in comparison with the free ligand (ppmh) (1.37 × 10 −6 S cm −1 versus 6.2 × 10 −7 S cm −1 ). The electrical conductivity is enhanced upon the irradiation of light. The specific detectivity of (89) is 11 times higher than that of the free ligand. All reported results suggest that this CP could be used for optoelectronic devices [102].

Conclusions
Relaying on the above-presented overview, it could be concluded that in recent years, a great number of Zn(II) carboxylate-based coordination polymers were developed by employing rigid carboxylate species alone or in combination with another rigid ligand, such as a heterocycle N-donor system. In these species, carboxylate ligands usually act as two, three or four-connectors, behavior related to the number of carboxylate groups, molar ratio, both substituents' nature and ancillary ligands. The structure and morphology of these materials is difficult to control since it depends on various factors, such as the stereochemical versatility of Zn(II), nature and coordinative ability of the carboxylate and ancillary ligand, molar ratio, synthesis method, solvent nature, temperature and sometimes even pH. The 1D, 2D or 3D structures thus developed allowed luminescence manifestation, an ability used for the sensing and determination of a large variety of water pollutants, both inorganic (Cr(VI), Mn(VII), Fe(III), Cd(II), Cu(II), Ni(II), and Al(III) species) and organic (antibiotics and nitroaromatic derivatives). On the other hand, photocatalytic and biological applications are related to the Lewis acid ability of the Zn(II) ion. Considering this, photocatalysts active in dye (MV, RhB, MR, MO, and MB) or drug (IBP) removal from wastewater solutions were developed. Additionally, this ability led to the design of some Zn-CBCPs as inhibitors of several pathogenic microorganisms (Gram-positive and Gram-negative bacteria, and fungi), as well as tumors (hepatic, gastro-intestinal or lymphoma), insensitive to platinum-based drugs. The pores generated inside the network are suitable to include gaseous (H 2 , N 2 , O 2 , CO, CO 2 , CH 4 , and C 2 H 2 ), liquid (1,2-dichloroethane, benzene, and cyclohexane) or solid (IBP and diclofenac sodium) species-generating materials valuable for storage, separation, or delivery. In conclusion, in this review we explored the vast and challenging domain of CPs and highlighted the main categories of properties for Zn-CBCPs, which are, in most cases, related with their structures.

Further Perspectives
The Zn-CBCPs described in this paper exhibit valuable properties and keep open the interest of researchers to obtain other similar species possibly with other applications that have been less addressed so far. As a result, the field can be developed both by using new polycarboxylate ligands with a rigid backbone and substituents with additional coordination ability, and by combining carboxylic derivatives with other polydentate ligands. The luminescent properties can be improved by including in the Zn-CBCPs network some known luminophores, such as lanthanide ions, for which several studies have been demonstrated so far, exhibiting their ability to modulate the optical properties of such derivatives. The domain of species with catalytic properties can be extended for other water pollutants and for other processes involving organic or inorganic species. Compounds with biological activity can also be tested for the inhibition of microbial biofilms or resistant tumors, aspects that have not been investigated so far in the field. In addition, the ability of some species to incorporate drugs could be explored for the development of another carrier, especially since the Zn(II) complexes are not toxic and have a low stability, thus easily releasing the active compounds. Finally, obtaining prosthetic materials or other opto-electrical properties can be further exploited in the future. The majority of the aforementioned species were synthesized mainly by the hydrothermal method, and it could be assumed that adopting other synthetic strategies may provide better control over the morphology. Therefore, even if in the past decade the domain of Zn-CBCPs was investigated in detail, the perspectives regarding their properties remain broad, with the possibility of improving existing ones or obtaining new candidates with new properties and applications. Hence, challenges still lie in the development of new species based on certain strategies which endow the CP with specific properties. The area of interest regarding the synthesis of new CPs will further gain importance as young scientists and researchers implement more efficient innovate methods with improved cost effectiveness.   TEA  triethylamine  TET  tetracycline  tib 1,3,5-tris(1-imidazolyl)benzene TNP 2,4,6-trinitrophenol TNT trinitrotoluene tpim 2,4,5-(tri(4-pyridyl)imidazole) tpom tetrakis(4-pyridyloxymethylene)methane trmb 1,3-bis(1,2,4-triazol-4-ylmethyl)benzene vim 1-vinylimidazole