Impact Factor : 0.548
Elena Florentina Grosu*1,2, Dana Simiuc3 and Renato Froidevaux1
Received: January 31, 2018; Published: February 22, 2018
*Corresponding author: Elena Florentina Grosu, department of Chemical engineering, Artois, University Littoral Cote d'Opale, EA 7394, ICV - Institut Charles Viollette, F-59000 Lille, France
Novel devices and materials are designed by exploring the nano scale properties of the matter. Nano and biomaterials can be combined together and further used as efficient tools in engineering, biomedicine, electronics and biotechnology. Layered double hydroxide, also called anionic clays, are a class of hydrotalcite nanocomposites with a large potential of utilization in nano and biotechnology. They have a high compositional diversity, are able of self-assembly and in presence of biomolecules they behave like soft materials, with no significant effect on their structure or activity. Some of the fields were the LDHs nanomaterials were successfully used are drug and gens delivery, cosmetics, cancer therapy and biosensing.
Keywords: Anionic clays; Drug delivery; Enzyme immobilization; Nanomaterials
Abbrevations: LDHs: Layered Double Hydroxides; DCF: Di-Clo-Fenac; BBC: Ber-Berine Chloride; CA: Cinnamic Acid; HRP: Horseradish Peroxidase; GCE: Glass Carbon Electrode; HBV: Hepatitis B virus
Nature is the most powerful and ingenious build maker. Piece by piece and step by step, nature created the largest source of inspiration and gave it to us with free and full access. Understanding how nature works at nanoscale level, humans started to imitate it and in this way different products and technologies were obtained and developed. Nanomaterials and nanotechnology are relatively new directions in research, being intensively approached by scientists in physics, chemistry, medicine, biology and engineering. Some of them are focused in improvement and development of new process and technologies, which can be used to obtain new materials with enhanced properties. In this context, nano-size materials are promising in fields such as environmental protection, electronics, medicine, electrical engineering, catalysis, biotechnology, energy, information technology, etc . The most important aspect regarding the nano form of the matter is that at scales ranging from individual atoms or molecules to submicron dimensions, the properties of matter change, and are very different from those of the same material, but in a bulk form. For example, some chemical elements like gold or silver are non-toxic in bulk form, but as nanoparticles are capable of killing viruses upon contact . In the last years, a class of inorganic materials, usually called anionic clays, has paid the attention of researchers from different fields. Layered double hydroxides (LDHs) are a nanocomposite class, whose organization is based on the structure of brucite (Mg (OH)2) in which a part of divalent cations have been replaced by trivalent cations . These materials are investigated due to their special properties and have technological importance in catalysis, drug delivery, medical science, cosmetics, biosensing, nanocomposite material engineering . This paper presents an overview on the most recently application of LDHs in biomedicine and biosensing.
Layered double hydroxides are known for over 150 years. The first discovered natural mineral with this structure was hydrotalcite , reported by Hochstetter in 1842, synthesized 100 years later by Feitknecht . The structure of this compound is related to that of brucite in which part of Mg2+ cations are replaced by Al3+. Carbonate anions are intercalated between the layers to maintain the electroneutrality of material. The presence of hydrotalcite in nature is reduced. For this reason, the synthesis of LDHs was investigated and was concluded that many transition metals in the first series can be used to obtain new materials with layered structure . Layered double hydroxides present a basic character and a great ability to intercalate anions in their structure . They have many of the physical and chemical properties of the mineral clay, being capable of ion exchange . The LDHs can be obtained by substitution of divalent cations in brucite with trivalent cations, so that the layers have a positive charge, which is balanced by intercalation of anions between layers. Also it is possible to vary the type and the ratio of di- and trivalent cations, as well as the type of anion in the interlayer space . Possibilities for obtaining LDHs structures with controllable properties, is one of the most attractive features of these compounds .
The general formula of LDHs materials is:
a) MII = divalent cation (Mg2+, Ca2+, Zn2+, etc.);
b) MIII = trivalent cation (Al3+, Cr3+, Fe3+, Co3+, etc.);
c) An- = interlayer anion with charge n- (Cl-, CO32-, NO3-,etc.);
d) x represents the molar ratio of [M (III) /(M (II) + M (III))];
e) y = number of water molecules located in the interlayer space [8,9].
Figure 1: Schematic representation of hydrotalcite structure.
Although the general formula involves only the presence of di- and trivalent cations, were obtained materials having a similar structure, but containing monovalent ions of lithium [10,11-15], In addition, was shown that it is possible to incorporate tetravalent ions in LDH structures. Thus, have been synthesized in (Figure 1) is presented the hydrotalcite structure. The interlayer space of the material is occupied by anions, water molecules and sometimes other neutral or charged moieties .The linkage of these species is carried out by a complex network of hydrogen bonding. The connection between the layers and interlayer space involves a combination of electrostatic effect and hydrogen bonding. Because of layered structure of LDHs, the anions must satisfy excess positive charge on both of the octahedral layers, which are electrically balanced by two neighboring interlayers. The interactions between the layers and interlayer galleries are realized particularly by strongly polarized hydroxyl groups which are bonded to trivalent cations and interact with the interlayer anions .
Biocompatibility: Recently, Cunha and co-worker elaborated a research on LDHs biocompatibility by testing two of the most popular LDHs materials, ZnAl-Cl and MgAl-Cl. In their protocol, rats were implanted with sterile LDHs materials, the clay particles being introduced in the specimen muscles. 28 days after implantation, the results show low toxicity and a very good biocompatibility of LDHs, with tissue reconstruction and no inflammation . In another study, rabbits were implanted with LDH. In the middle ear of the specimens were introduced MgAlLDH and the health animals evolution was monitored during 10 days. The material was highly biocompatible and slowly dissolved, being a promising nanocomposite for drug delivery .
Drug Delivery: One particular property of LDHs nanocomposite is their ability to incorporate large molecule as drugs. Furthermore, they can increase the drug solubility without the alteration of pharmaceutical therapeutic activity . In a recent study, Dejbbi and collaborators prepared via co-precipitation and ionic- exchange a drug-LDH hybrid between berberine chloride (BBC) and MgAlLDH. For both materials, denoted as BMAE (obtained via ionic exchange) and BMAC (obtained via co-cipitation), the drug was not incorporated in the LDH interlayer space, being adsorbed on clay surface. The release of BBC from biohybrids was carried out in PBS buffer, pH 7.4, 37 °C. After 50 minutes, the equilibrium for the drug release is reached. Even if the reaction continued till 200 minutes, the maximum BBC release was 63 % and 87 % for BMAE and BMAC, respectively. In order to evaluate how the LDH matrix is affecting the therapeutic activity of BBC, in vitro tests were performed. Different pathogen bacteria were put in contact with the BBC-LDHs biohybrids, for 24 h. The results show that the bionanocomposites have anti-bacterial activity, and they can inhibit the pathogen proliferation .
Diclofenac (DCF) is an anti-inflammatory drug which was intercalated in the interlayer space of ZnAlLDH, through an ion- exchange method. The DCF release had its maximum of 84 % after cca. 600 minutes of stirring, at pH 7.4; the tests were carried out in phosphate buffer solutions with pH 7.4 and 4.8, 37 °C . Recently, efavirnez (EFZ), a pharmaceutical used as inhibitor for Human Immunodeficiency Virus, has successfully adsorbed on CaAlLDH. The EFZ drug is characterized by a poor solubility in water. However, when released from the LDHs surface, the drug solubility increases with 558 % compared to the EFZ alone, after two hours. The cytotoxicity tests proved that CaAlLDH has a small effect on cells viability . Vitamin C was also incorporated in CaAlLDH and released in phosphate buffer, pH 7.4 . Other drugs which were coupled with layered double hydroxides are given in Table 1, together with some observation related on the way how the complex is formed (drug adsorption or interlayer intercalation) and the type of study [24-34].
Table 1: LDHs used in biomedical application.
Other LDHs Application in Biomedicine: Carja research group  obtained self-assembled gold nanoparticles (AuNPs) on LDHs matrix. The resulted hybrids were tested against hepatitis B virus (HBV). Firstly, three type of LDH matrix were synthesized: MgAlLDH, ZnAlLDH and MgFeLDH. Furthermore, AuNPs/LDHs hybrids were obtained by exploring the structural memory effect of the anionic clays. For all the AuNPs/LDHs nanocomposite cytotoxicity and the antiviral activity were monitored. Results have shown that the hybrids have an inhibitory effect on HBV proliferation and a good cytocompatibility.Negatively charged organic molecules as DNA has been adsorbed on LDH by electrostatic binding  In a previews study done by Balcomb et al. in 2015 has been proved that the proprieties of LDHs nanocomposite (MgAlLDH, ZnAlLDH, MgFeLDH and ZnFeLDH) recommend them as DNA delivery vector for mammalian cells transfection. In vitro tests carried out on hepatocellular carcinoma, embryonic kidney, and cervical cancer cell lines show low cytotoxic activity of LDH materials .
Another direction ofLDH materials application is their utilization in cosmetics, especially for sunscreen formulation production . The main issue of the existent sunscreen products is their organic content, which besides the UV absorption properties, they penetrate the skin and undesirable effects can occur. To avoid this, the organic compounds might be adsorbed on an inorganic matrix . Some of the LDHs materials are excellent in UV radiation absorption . For this reason, the incorporation of organic molecules on clays structure can lead to a synergic effect in UV absorption. Recently, Li et al. intercalated in ZnTiLDH matrix the cinnamic acid (CA) and they evaluated the UV absorption properties. The results have proven that after intercalation, the resulted material (ZnTi-CA-LDH) presents enhanced capacity of UV absorption. Furthermore, the hybrid was used for a cosmetic cream fabrication and the UV absorption capacity was measured and compared with a blank sample, without any ZnTi-CA-LDH in its composition.
The UV-Vis measurements have shown that the addition of the new material in the sun cream increases significantly the UV absorption ability of the final product . Significant increase of UV radiation absorbance was observed for the hybrid resulted from the intercalation of benzophenone (B9) into ZnAlLDH matrix. Furthermore, in vitro studies were performed and the results have shown that the ZnAlLDH-B9 material had no significant cytotoxic effect on the human dermal fibroblasts cells .
Layered Double Hydroxide in (bio) Sensing: Layered double hydroxides are nanomaterials with high stability, low toxicity, variable morphological properties and significant adsorption capacity. Due to these characteristics, LDHs were perfect candidates for the immobilization of different molecules and biomolecules as DNA, drugs and enzymes . Furthermore, they can be considerate as promising materials for electrochemical sensors and biosensor design.
Enzyme-LDHs as Biohybrid in Biosensors Design: The enzymes are biomolecules with catalytic activity which can be used in different processes due to their high specificity and efficiency . LDHs nanocomposites have proven to be suitable supports for enzyme immobilization, without notable modification of enzyme activity . One of the most important directions of enzyme immobilization on LDHs nanocomposites is their utilization in biosensing. Recently, tyrosinase enzyme was supported on CoAlLDH in order to obtain a biosensor able to detect the polyphenols, especially from tea. A robust polyphenols biosensor was designed by covering gold electrodes with LDH, followed by tyrosinase binding to the modified electrodes. The biosensor was used many times with good performances, and it is appropriate to be used in fields where traces need to be detected (medicine, environment) . Horseradish peroxidase (HRP) is an oxidoreductase used in biosensig for H2O2 detection. In 2015 Wang designed a H2O2 biosensor by attaching to a glass carbon electrode (GCE) a biohybride containing HRP immobilized on a mixture of carbon nanodots (C-Dots) and CoFeLDH.
The new biosensor reached its maximum performance at pH 7 and it had superior analytical performances in terms of selectivity and detection limit in comparison with C-Dots/GCE, CoFeLDH/GCE, C-Dots/CoFeLDH/GCE formulation . HRP was also immobilized on MgAlLDH , NiAlLDH . Furthermore, horseradish peroxidase and glucose oxidase (GOX) were immobilized together on ZnCrLDH-ABTS composite. The bienzymatic biosensor was used for glucose determination. The operation principle involved the formation of H2O2 as a product of glucose oxidation (reaction catalyzed by GOx), followed by HRP oxidation in presence ofhydrogen peroxide, and HRP reduction by ABTS . Other formulations for biosensors design were obtained from MgAlLDH [48,49], ZnAlLDH  and alkaline phosphatase for hydroquinone determination, MgAlLDH and acetylcholinesterase for organophosphate pesticides detection . Also, for biosensing purpose, cellulase was immobilized on MgAlLDH . Hemoglobin (Hb), which is an iron- containing protein, was immobilized on exfoliated CoAlLDH  and ZnAlLDH . Both Hb-LDH biosensors were used successfully for trichloroacetic acid biosensing, with high sensitivity and low detection limit.
LDHs in Non-Enzymatic Sensors Design: Beside the incorporation of active biomolecules into their structure, LDHs nanocomposites can be used alone or in combination with non-enzymatic materials for sensors fabrication. Particularly, hydrotalcites containing transition metals (e.g. Mn, Ni, Co) are considerate as promising materials in electrochemistry . Last year, Zhan et al. designed a sensor for bisphenol A (BPA) quantification . For this, exfoliated NiAlLDH was deposited on glass carbon electrode (GCE) and used in BPA determination from milk. Sensor reproducibility, stability and selectivity were investigated. The results have shown that the product is highly selective, stable, having an excellent repeatability . NiAlLDH was also the subject for acetylcholine (ACh) sensor design. Enhanced sensor electroconductivity was obtained when NiAlLDH material was combined with carbon dots .
Diabetes is a disease with a constantly increase of patients number. Efforts are focused on developing instruments and methods for rapid and accurate detection of glucose levels in human blood . Glucose oxidase free sensors were intensely approached in the last years, because of the inconvenience like enzyme low stability and deactivation due to pH and temperature variation . LDHs materials were used in non-enzymatic glucose detection. However, because of their low conductivity, the addition of carbon containing materials can solve this issue. Recently, Samuei et al. fabricated a graphene quantum dots - CoNiAlLDH sensor successfully used for glucose measurements from human blood sample . Previously, NiCoLDH-carbon cloth  and Au-NiAlLDH-carbon nanotubes- graphen  sensor were designed for the same purpose. Furthermore, NiAlLDH attached on a carbon ionic liquid electrode was used for dopamine oxidation and quantification .
In this paper, layered double hydroxides applications in biomedicine and (bio) sensing were presented. Due to their proprieties as high physico-chemical stability, low toxicity and good compatibility with the living cells, the LDHs materials have been found as good candidates for drug delivery, sunscreen products fabrication and active biomolecules incorporation. In vivo and in vitro studies were done and the results have shown that the anionic clays have low cytotoxic effect, being able to increase the therapeutic activity and the solubility of some drugs. Furthermore, LDHs gold containing presented an inhibitory effect for hepatitis B virus. Combined with enzyme or other non-enzymatic materials, the anionic clays were used for the fabrication of electrochemical sensors. It might be concluded that the layered double hydroxides are a class of biomaterials with a large potential in drug delivery, cancer therapy and bioimaging technology.
Modificatiom in Layered Double Hydroxides: Present and Future, Nova Science, New York, USA.