Recent Advances in Calixarene-Based Fluorescent Sensors for Biological Applications

Due to their structural features, macrocyclic compounds such as calixarenes, conjugated with a variety of fluorophores have led to the development of fluorescent probes for numerous applications. This review covers the recent advances (from 2009 to date) made in calixarene-based fluorescent sensors and their biological applications. In addition to the fluorescence mechanisms used to signal the analyte binding, this article focuses mainly on the detection of biological relevant ions, on the selective sensing of biomolecules, such as amino acids, enzymes, drugs and other organic compounds, and on intracellular imaging. Calixarene-containing fluorescent nanoparticles and nanoaggregates for imaging and drug delivery are also described. Finally, this review presents some conclusions and future perspectives in this field.

Self-Assembled Plasmonic Nanojunctions Mediated by Host-Guest Interaction for Ultrasensitive Dual-Mode Detection of Cholesterol

Herein, a fluorescence and surface-enhanced Raman spectroscopy dual-mode system was designed for cholesterol detection based on self-assembled plasmonic nanojunctions mediated by the competition of rhodamine 6G (R6G) and cholesterol with β-cyclodextrin modified on gold nanoparticles (HS-β-CD@Au). The fluorescence of R6G was quenched by HS-β-CD@Au due to the fluorescence resonance energy transfer effect. When cholesterol was introduced as the competitive guest, R6G in the cavities of HS-β-CD@Au was displaced to recover its fluorescence. Moreover, two of HS-β-CD@Au can be linked by one cholesterol to form a more stable 2:1 complex, and then, plasmonic nanojunctions were generated, which resulted in the increasing SERS signal of R6G. In addition, fluorescence and SERS intensity of R6G increased linearly with the increase in the cholesterol concentrations with the limits of detection of 95 and 74 nM, respectively. Furthermore, the dual-mode strategy can realize the reliable and sensitive detection of cholesterol in the serum with good accuracy, and two sets of data can mutually validate each other, which demonstrated great application prospects in the surveillance of diseases related with cholesterol.

Electrochemical sensor for human norovirus based on covalent organic framework/pillararene heterosupramolecular nanocomposites

Noroviruses are the leading cause of acute gastroenteritis and food-borne diseases worldwide. Thus, a rapid, accurate, and easy-to-implement detection method for controlling infection and monitoring progression is urgently needed. In this study, we constructed a novel sandwich-type electrochemical biosensor integrated with two specific recognition elements (aptamer and peptide) for human norovirus (HuNoV). The electrochemical biosensor was fabricated using magnetic covalent organic framework/pillararene heterosupramolecular nanocomposites (MB@Apt@WP5A@Au@COF@Fe₃O₄) as the signal probes. The sensor showed high accuracy and selectivity. The detection method does not need the extraction and amplification of virus nucleic acid and has a short turn-around time. Intriguingly, the proposed biosensor had a limit of detection of 0.84 copy mL^-1 for HuNoV, which was the highest sensitivity among published assays. The proposed biosensor showed higher sensitivity and accuracy compared with immunochromatographic assay in the detection of 98 clinical specimens. The biosensor was capable of determining the predominant infection strain of GII.4 and also GII.3 and achieved 74% selectivity for HuNoV GII group. This study provides a potential method for point-of-care testing and highlights the integrated utilization of Apt and peptide in sensor construction.

Calix[n]arene/Pillar[n]arene-Functionalized Graphene Nanocomposites and Their Applications

Calix[n]arenes and pillar[n]arenes, which contain repeating units of phenol and methane, are class of synthetic cyclic supramolecules. Their rigid structure, tunable cavity size, flexible functionalization, and rich host-guest properties make them ideal surface modifiers to construct functional hybrid materials. Introduction of the calix[n]arene/pillar[n]arene species to the graphene may bring new interesting or enhanced physicochemical/biological properties by combining their individual characteristics. Reported methods for the surface modification of graphene with calix[n]arene/pillar[n]arene utilize either covalent or non-covalent approaches. This mini-review presents the recent advancements in the functionalization of graphene nanomaterials with calix[n]arene/pillar[n]arene and their applications. At the end, the future outlook and challenges for the continued research of calix[n]arene/pillar[n]arene-functionalized graphene nanohybrids in the development of applied nanoscience are thoroughly discussed.

A novel fluorescence assay for resveratrol determination in red wine based on competitive host-guest recognition

A label-free fluorescence assay for resveratrol determination is presented for the first time. The approach was based on fluorescence resonance energy transfer (FRET), via competitive supramolecular recognition, between p-sulfonated calix[6]arene (CX6)-modified reduced graphene oxide (CX6@RGO) and a probe-resveratrol complex. The probe molecule (Rhodamine B or rhodamine 123) had a strong fluorescence signal, and its fluorescence was quenched by CX6@RGO, based on FRET. When the target molecule was added to CX6@RGO, the probe molecule was displaced by resveratrol, and a host-guest complex, CX6@RGO-resveratrol formed, turning-on the fluorescence signal. Fluorescence intensity of the CX6@RGO-probe complex increased linearly with increased resveratrol concentrations (2.0-40.0 μM). The proposed approach was used to determine resveratrol in red wine with satisfactory detection limits and recoveries. Compared with traditional determination methods, our procedure is advantageous because it saves time, is easy to operate, and does not require sample pretreatment.

Fluorescence Resonance Energy Transfer Systems in Supramolecular Macrocyclic Chemistry

The fabrication of smart materials is gradually becoming a research focus in nanotechnology and materials science. An important criterion of smart materials is the capacity of stimuli-responsiveness, while another lies in selective recognition. Accordingly, supramolecular host-guest chemistry has proven a promising support for building intelligent, responsive systems; hence, synthetic macrocyclic hosts, such as calixarenes, cucurbiturils, cyclodextrins, and pillararenes, have been used as ideal building blocks. Meanwhile, manipulating and harnessing light artificially is always an intensive attempt for scientists in order to meet the urgent demands of technological developments. Fluorescence resonance energy transfer (FRET), known as a well-studied luminescent activity and also a powerful tool in spectroscopic area, has been investigated from various facets, of which the application range has been broadly expanded. In this review, the innovative collaboration between FRET and supramolecular macrocyclic chemistry will be presented and depicted with typical examples. Facilitated by the dynamic features of supramolecular macrocyclic motifs, a large variety of FRET systems have been designed and organized, resulting in promising optical materials with potential for applications in protein assembly, enzyme assays, diagnosis, drug delivery monitoring, sensing, photosynthesis mimicking and chemical encryption.

Ultrasensitive electrochemical detection of Dicer1 3'UTR for the fast analysis of alternative cleavage and polyadenylation

Alternative cleavage and polyadenylation (APA) is involved in several important biological processes in animals, e.g. cell growth and development, and cancer progression. The increasing data show that cancer cells are inclined to produce mRNA isoforms with a shortened 3'UTR undergoing APA. For example, the Dicer1 isoform with a shorter 3'untranslated region (3'UTR) was found to be overexpressed in some cancer cells, which may be used as a potential novel prognostic biomarker for cancer. In the present work, a novel electrochemical biosensor for ultrasensitive determination of Dicer1 was designed by using gold nanoparticles and p-sulfonated calix[6]arene functionalized reduced graphene oxide (Au@SCX6-rGO) as nanocarriers. The results showed that the expressions of the shorter 3'UTR (Dicer1-S) both in BT474 and SKBR3 were obviously higher than those of the longer Dicer1 (Dicer1-L) by the constructed biosensor, which agreed well with the result analyzed by the RT-qPCR method. The detection ranges of Dicer1-S and Dicer1-L were 10^-14-10^-9 M and 10^-15-10^-10 M. The LODs were 3.5 and 0.53 fM. The specificity of the proposed biosensor was also very high. For the first time, the expressional analysis of different 3'UTRs caused by APA was studied by an electrochemical method. Moreover, the use of a macrocyclic host for constructing an electrochemical/biosensing platform has rarely been reported. The proposed electrochemical sensing strategy is thus expected to provide a new method for determination of novel biomarkers and a novel method for fast and cheap analysis of APA.