Luminescent films for chemo- and biosensing

Luminescent films have received great interest for chemo-/bio-sensing applications due to their distinct advantages over solution-based probes, such as good stability and portability, tunable shape and size, non-invasion, real-time detection, extensive suitability in gas/vapor sensing, and recycling. On the other hand, they can achieve selective and sensitive detection of chemical/biological species using special luminophores with a recognition moiety or the assembly of common luminophores and functional materials. Nowadays, the extensively used assembly techniques include drop-casting/spin-coating, Langmuir-Blodgett (LB), self-assembled monolayers (SAMs), layer-by-layer (LBL), and electrospinning. Therefore, this review summarizes the recent advances in luminescent films with these assembly techniques and their applications in chemo-/bio-sensing. We mainly focused on the discussion of the relationship between the sensing properties of the films and their architecture. Furthermore, we discussed some critical challenges existing in this field and possible solutions that have been or are being developed to overcome these challenges.

Bioinspired design and assembly of layered double hydroxide/poly(vinyl alcohol) film with high mechanical performance

Inspired by the hierarchical structure and excellent mechanical performance of nacre, LDH nanosheets with an appropriate aspect ratio to withstand significant loads and at the same time allow for rupture under the pull-out mode were synthesized as artificial building blocks for the fabrication of nacre-like films. Multilayered PVA/LDH films with a high tensile strength and ductility were prepared for the first time by bottom-up layer-by-layer assembly of pretreated LDH nanosheets and spin-coating of PVA. The weight fraction of inorganic LDH platelets in the hybrid PVA/LDH films (wp) was controlled by changing the concentration of PVA solution applied in the spin-coating process. The resulting films revealed that the PVA/LDH hybrid films were piled close together to form a well-defined stratified structure resembling the brick-and-mortar structure of natural nacre. In the hybrid films, the content of inorganic LDH platelets was comparable to the value in nacre, up to 96.9 wt %. It could be clearly seen that the mechanical performance of the as-prepared PVA/LDH films was greatly improved by increasing the rigid building-block LDHs. The tensile strength of the 2 wt % PVA/LDH hybrid film reached a value of 169.36 MPa, thus exceeding the strength of natural nacre and reaching 4 times that of a pure PVA film. Meanwhile, its elastic modulus was comparable to that of lamellar bone.

Tuning solid-state blue and red luminescence by the formation of solvate crystals

Tuning and controlling the solid-state luminescence of molecular solids play a key role in developing multi-color displays and tunable dye laser. In this work, we report the tunable blue and red luminescence by the formation of solvate crystals of 1,4-bis(5-phenyl-2-oxazolyl)benzene (POPOP) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). Upon introduction of guest solvents (chloroform and dichloromethane) into the POPOP and DCM host matrices, the obtained solvate crystals exhibit an alternated stacking arrangement, interaction fashion, and crystal symmetry compared with the pristine chromophore solids. Furthermore, the solvates of POPOP (CCl3H) and DCM (CCl2H2) present changeable luminescent properties (such as one-/two-photon emissive wavelength, fluorescence lifetime and photoluminescent quantum yield) in the blue/red regions relative to the pristine POPOP and DCM. In addition, the second harmonic generation can also be obtained for the DCM (CCl2H2) due to the transformation of the centrosymmetric to a non-centrosymmetric structure from pristine DCM. Periodic density functional theoretical calculations suggest that the guest solvents do not participate in the frontier orbital distribution within the solvate crystals. Therefore, by the combination of experimental and theoretical studies on the solvate crystals, this work not only reports the supramolecular assembly of new types of host-guest photoactive systems, but also provides a detailed understanding of the electronic structures of the solid-state luminescent materials.