Bio-inorganic Composite Films Based On Layered Double Hydroxides: Construction And Potential Application As Fluorescent Biosensor

Layered double hydroxides (LDHs) is one kind of important two-dimensional nanomaterial. Recently, with the development of inorganic material, the 2D film based on LDH, which has unique physical and chemical properties, exhibit a broad application prospect in many fields. Previous works have realized the assembly of different organic and inorganic functional molecules with LDHs nanosheets based on electrostatic interactions and hydrogen bonding interaction, and constructed a series of composite films with various nature and function.However, there is few report about other guest molecules and driving force to construct LDHs based films, which significantly limits the development of LDHs based films with diversity application. Therefore,it is an important research direction and challenge to explore and develop other assembly driving force, and achieve more types of functional units for the building of LDHs based films to meet specific requirements in various applications. In this thesis, LDHs nanosheets were selected as the host, biological macromolecules or small molecules were selected as the guests, a series of bio-inorganic nanocomposite luminescent films were constructed successfully based on electrostatic interaction, hydrogen bonding and van der Waals force. The properties of the guest molecules in the composite films were improved due to the two-dimensional LDHs structure, which also achieved the immobilization of the biological molecules and small molecules. Based on their excellent fluorescent properties, these composite films would be a novel type of potential biological sensors. This work has made a beneficial exploration for the preparation of bio-inorganic composite films and its application in biological fluorescence sensing. The main contents of this paper are as follows:1. The small anion 8-anilino-l-naphthalenesulfonate (ANS) was successfully assembled with LDH nanosheets to form (ANS/LDH)n ultrathin films (UTFs) by using the layer-by-layer (LbL) assembly technique. The drive force of the (ANS/LDH)n system was discussed and attributed to electrostatic interaction and two possible weak interactions:hydrogen-bond and induced electrostatic interaction between ANS and positive-charged LDH nanosheets. The obtained (ANS/LDH)n UTFs show uniform and long-range-ordered periodic layered structure. These UTFs can fluorescently response of different polarity environment reversibly, and the original broad fluorescence peak split into three induced by the 2D confinement effect of the LDH monolayers and the change of the polar environment. Meanwhile, the composite UTFs exhibit selective fluorescence enhancement in the presence of hydrophobic BSA-like protein biomolecules, and the rate of fluorescence enhancement with the concentration is significantly different with the different BSA aggregation state, which may be used not only to detect the protein concentration but also to monitor the aggregation states of the BSA-like protein. This work realized the assembly of the small molecule with special structure and LDHs nanosheets based on three kinds of driving force, constructed a novel kind of fluorescence composite film with excellent performance.2. A kind of bio-inorganic nanocomposite luminescent films was fabricated by the alternate assembly of enhanced green fluorescent protein (EGFP) and LDH nanosheets based on electrostatic and hydrogen-bond interactions, which realized the immobilization of EGFP.The obtained (EGFP/LDH)n UTFs show long-range-ordered periodic layered stacking structure and strong fluorescence originating from EGFP.The inorganic LDH laminates provided a well hydrophilic environment for the EGFP molecules, which played an important role to protect and improve the structure and properties of the EGFP in the UTFs. The(EGFP/LDH)n UTFs exhibited a reversible fluorescence response between different pH value (4-8), which is due to the changes of protonated states of the EGFP chromophore. At the same time, the(EGFP/LDH)n UTFs have significant different fluorescence intensity for the wet and dry environment, which is useful to further study the effect of water molecule on the fluorescence of EGFP. Furthermore, the UTFs could detect some small biological medicine molecules, such as protoporphyrin, through inner-filter effect. This work shows great promise for developing advanced bio-hybrid films with well-organized structures and controllable functions, a crucial requirement for advanced sensing applications.3.The small molecular dye 4’,6-diamidino-2-phenylindole (DAPI)blending with ssDNA were co-assembled with LDH nanosheets to form(ssDNA@DAPI/LDH)n UTFs by the LbL assembly technique, which realized the immobilization of biomolecule ssDNA and DAPI. The obtained (ssDNA@DAPI/LDH)n UTFs show uniform and long-range-ordered periodic layered structure. This UTFs can sensor fluorescence enhancement of the complementary ssDNA sequence and have good selectivity for the long complementary ssDNA sequence over a range of other ssDNA sequence. It was speculated that the detection mechanism is attributed to the formation of double helix structure by the interaction with complementary ssDNA sequence and the groove binding mode between DAPI and the double helix structure. In addition, the(ssDNA@DAPI/LDH)n UTFs displayed a well stability and reversiblity fluorescence response for the complementary ssDNA sequence, which indicates that this film can be a novel potential fluorescence sensor for the specific ssDNA sequence.4. Fluorescence enhanced hybrid materials DES-BP4(x%)/LDH were successfully prepared by co-intercalating small anion 2-hydroxy-4-methoxybenzophenone-5-sulfonate (BP4) and decanesulfonic acid sodium salt (DES) with different molar rations into LDHs via the hydrothermal and solution co-precipitation methods. The fluorescence properties of the guest BP4 in the hybrid materials were improved prominently due to the 2D confinement effect of the LDHs nanosheets. Its thin film on a quartz substrate is obtained by the solvent evaporation method, which displayed well fluorescence properties. The film exhibited a reversible fluorescence response between different pH value (4-8), which is due to the changes of protonated states of BP4. At the same time, the film could detect small biological medicine molecule protoporphyrin through inner-filter effect. Therefore, the film is a novel type of potential biological sensor for pH or small biological molecules.