Toward High Sensitivity: Perspective on Colorimetric Photonic Crystal Sensors

Enzyme-containing-hydrogel/TiO2 hybrid photonic crystal for label-free detection of small molecules

Label-free optical biosensing is a promising key technology in medical diagnosis and biomedical science. The refractive-index (RI) sensitivity of photonic-crystal (PhC)-based biosensors can facilitate rapid, simple, and label-free sensing of biomolecules, with a compact measurement setup. However, the top-down fabrication of highly reproducible PhCs is expensive. Moreover, their RI sensitivity is not sufficient to detect the RI changes caused by small molecules such as glucose. Here, we propose a highly reproducible enzyme-containing-hydrogel/TiO2 hybrid-PhC-based sensing system for label-free small-molecule detection. Our hybrid PhC can detect small molecules by utilizing the reflection-spectrum responses of TiO2-based PhCs to the RI changes of the hydrogel on the surface, caused by the shrinking and swelling of the hydrogel in response to a reaction between the immobilized enzyme and target substrate. In this report, we demonstrate the rapid and simple detection of glucose, using a GOx-containing hybrid PhC. Our hybrid PhC can selectively and rapidly detect 1 nM glucose, with a compact setup, from the shrinking of the hydrogel caused by the oxidation of glucose by GOx. Our sensing system is expected to pave the way for the application of PhC-based biosensors in the detection of small molecules.

Towards injured joint rehabilitation: structural color hydrogels for accelerated wound healing and rehabilitation exercise monitoring

Joint injuries caused by severe acute trauma seriously affect patients' mobility and quality of life. Traumatic or postoperative wound healing and rehabilitation training are both essential for restoring joint functions, calling for effective wound healing materials that are also capable of monitoring rehabilitation training for joint condition evaluation and physical therapy guiding. Herein, a structural color hydrogel for wound care and naked-eye rehabilitation exercise monitoring of injured joints is designed by constructing a hybrid double-network, which contains a covalently crosslinked network and a Zn2+ coordination based dynamic network. The crosslinking formed by Zn2+ coordination endows the structural color hydrogel with enhanced mechanical properties for joint wounds with motion requirements, as well as antibacterial, anti-inflammatory, and pro-angiogenic properties that promote wound healing. Meanwhile, the Poisson's ratio of the structural color hydrogel can be easily tuned by varying the covalently-crosslink density to achieve sensibility ranging from 3.6 nm to 6.2 nm photonic-bandgap shift per 1% strain, achieving a remarkable color change responding to joint range-of-motion from minimal (0-2°) to wide-range (0-90°) bending during rehabilitation exercises. This structural color hydrogel provides an approach to the multi-stage management of joint injuries and real-time clinical insights into rehabilitation progress.

A portability self-powered sensor facilitates sensitive Cd2+ detection: Dual mechanism and three quantitative mode

As a common heavy metal contaminant, Cd2+ has adverse effects on food safety and consumer health. It is very important for human health to realize highly sensitive Cd2+ detection methods. The self-powered sensing system based on enzyme biofuel cells (EBFCs) does not need an external power supply, which can simplify the experimental equipment and has great application value in portable detection. Thus, the biosensor is innovatively integrated into the screen-printed electrode to construct a new type of portable sensor suitable for on-site and real-time Cd2+ detection. Hybridization chain reaction (HCR) combined with the Cd2+-dependent deoxyribose (DNAzyme) signal amplification strategy is used to enhance the detection sensitivity while specifically recognizing the Cd2+. Moreover, the self-powered sensor combines with smartphones to realize quantitative Cd2+ detection without other instruments and has the characteristic of Effectively improving the hazard detection technology is essential to ensure food safety. Portability, simplicity, and speed are suitable for real-time Cd2+ detection in the field. The dual mechanism and three quantitative modes combining colorimetric and two electrical signals output modes are adopted to realize the visualization and accurate detection. A series of research results confirm that this strategy is of great significance to strengthen the development of intelligent Cd2+ technology, expand the application of self-powered sensing technology, and improve the safety detection system.

β-Hydroxybutyrate dehydrogenase functionalized two-dimensional photonic crystals for quantitative and visual sensing of ketone bodies

β-Hydroxybutyrate (BHB) is a substantial physiological ketone body. Its elevated concentration causes ketoacidosis, which is a disorder with a high mortality rate. Therefore, there is an urgent need to develop a simple method for the in-situ monitoring of BHB in urine. In this study, a photonic crystal hydrogel (PCH) sensing material for the detection of urinary ketones was prepared by embedding a two-dimensional polystyrene photonic crystal array (PCA) in a hydrogel functionalized with β-hydroxybutyrate dehydrogenase (BHBDH). BHBDH catalyzes the interconversion between β-hydroxybutyrate and acetoacetic acid and relies on the cofactor nicotinamide adenine dinucleotide (NAD+) to participate in the reaction process. The catalytic cycle of converting β-hydroxybutyrate to acetoacetate generates H+, which reduces the electrostatic repulsion between the carboxyl groups in the hydrogel network, ultimately leading to the shrinkage of the hydrogel volume. The hydrogel volume change was detected by measuring the diameter of the Debye diffraction ring, thus reflecting the concentration of BHB. When the concentration of BHB was increased from 0 to 10 mM, the reflection spectrum of PCH shifted for 117 nm within 60 min, consequently, the structural color of PCH changed from red to green and finally to blue. The material was used for quantitative detection of BHB with a detection limit of 48.94 μM. Then it was used for detection in artificial urine samples. While, this smart and reusable sensing material could provide a more convenient and efficient strategy for the ketone body detection in clinical diagnosis and point-of-care monitoring.

Hydrogel Grating Sensors with Boron Affinity and Molecular Imprinting Effects for Rapid and Sensitive Detection of Tumor Marker Sialic Acid

Rapid and sensitive detection of the concentration of sialic acid (SA) in serum is crucial for early tumor screening and prognostic assessment; however, it still remains challenging. Here, we propose a novel kind of hydrogel grating sensor with boron affinity and molecular imprinting effects (B-MIP) for the rapid and sensitive detection of SA concentration in serum. The hydrogel gratings feature uniform surface relief microstructures and incorporate highly specific recognition binding sites into SA molecules provided by boron affinity and molecular imprinting. The periodic nanoridges of hydrogel gratings increase the specific surface area contacting the environmental solution; therefore, fast detection can be achieved within 2 min. Upon recognition of SA molecules, the height of hydrogel gratings changes at the nanoscale, causing a change in the diffraction efficiency of the hydrogel gratings. The B-MIP hydrogel grating sensors have highly specific binding sites to SA molecules distributed throughout the whole hydrogel and can preferentially and selectively recognize and respond to the SA molecules even in the presence of interference substances glucose and fructose with high concentrations. The B-MIP hydrogel grating sensors are effectively applicable for the rapid and sensitive detection of SA concentrations in real serum samples with satisfactory accuracy and precision. Our approach provides an excellent strategy to address the current challenges in SA detection and provides new insights into the detection of tumor markers in serum, thereby opening up new ways to accurately detect complex biological samples in analytical science.

Colloidal photonic crystals towards biological applications

Colloidal photonic crystals (CPCs), fabricated from the assembly of micro-/nano-particles, have attracted considerable interest due to their unique properties, such as structural color, slow-photon effect, and high specific surface area (SSA). Benefiting from these properties, significant progress has been made in the biological applications of CPCs. In this perspective, these properties and relative manipulation strategies are firstly discussed, building bridges between properties and biological applications of CPCs. Structural color endows CPCs with naked-eye sensing capability, which can be applied to physiological state assessment and diagnosis, as well as self-report of CPC-based diagnostic and therapeutic devices. The slow-photon effect contributes to enhanced fluorescence, surface-enhanced Raman scattering, and efficacy of photodynamic/photothermal therapy, when CPCs are combined with corresponding functional materials. High SSA provides CPCs with abundant binding sites and superior capabilities for loading, adsorption, delivery, etc. These properties can be utilized individually or synergistically to grant CPCs superior performance in biological applications. Next, the recent advancements of CPCs towards biological applications are summarized, including biosensors, wound dressings, cells-on-a-chip, and phototherapy. Finally, a perspective on the challenges and future development of CPCs for biological applications is presented.

Recent advances in photonic crystal-based chemical sensors

The increasing attention towards environmental quality, food safety, public security and medical diagnosis demands high requirements and standards for chemical sensors with merits of rapid response, high precision, long-term stability and reusability. In this case, a prominent innovation in sensory materials holds potential to realize new generations of chemical sensor technologies. Specifically, photonic crystals (PCs) as structured dielectric materials with spatially periodic ordered arrangements offer unique advantages in improving the sensing performance of chemical sensors. Consequently, the promising properties of PCs promote research on their implementation as an integral part of chemical sensors. This review highlights the integration of PCs into chemical sensors including a range of building blocks for the construction of PCs with versatile opal or opal inverse structural architectures and a delicate choice of surface functionality with associated sensing interfaces for target recognition and signal transduction. Subsequently, based on their synthesis and functionality, we focus on introducing the sensing principles of recent advances in PC-based chemical sensors, such as reflection spectra-based sensing, visual colorimetric sensing, fluorescence sensing, surface-enhanced Raman spectroscopy (SERS)-based sensing and other integrated sensing. Finally, the future prospects and challenges are discussed for the further improvement of PC-based chemical sensors.

Self-assembly of cascade nanoenzyme glucose oxidase encapsulated in copper benzenedicarboxylate for wearable sweat-glucose colorimetric sensors with smartphone readout

BACKGROUND

With the advent of personalized medical approaches, precise and tailored treatments are expected to become widely accepted for the prevention and treatment of diabetes. Paper-based colorimetric sensors that function in combination with smartphones have been rapidly developed in recent years because it does not require additional equipment and is inexpensive and easy to perform. In this study, we developed a portable, low-cost, and wearable sweat-glucose detection device for in situ detection.

RESULTS

The sensor adopted an integrated biomimetic nanoenzyme of glucose oxidase (GOx) encapsulated in copper 1, 4-benzenedicarboxylate (CuBDC) (GOx@CuBDC) through a biomimetic mineralization process. CuBDC exhibited a peroxide-like effect, cascade catalytic effect with the encapsulated GOx, and increased the enzyme stability. GOx@CuBDC and 3,3,5,5-tetramethylbenzidine were combined to form a hybrid membrane that achieved single-step paper-based glucose detection.

SIGNIFICANCE AND NOVELTY

This GOx@CuBDC-based colorimetric glucose sensor was used to quantitatively analyze the sweat-glucose concentration with smartphone readings. The sensor exhibited a good linear relationship over the concentration range of 40-900 μM and a limit of detection of 20.7 μM (S/N = 3). Moreover, the sensor performed well in situ monitoring and in evaluating variations based on the consumption of foods with different glycemic indices. Therefore, the fabricated wearable sweat-glucose sensors exhibited optimal practical application performance.

Target Recognition Initiated Self-Assembly-Based Signal Amplification Strategy for Sensitive and Colorimetric Staphylococcus aureus Detection and Diagnosis of Skin Infection

Staphylococcus aureus (S. aureus), as a Gram-positive bacterium, is commonly encountered in various infectious diseases, such as acute skin and soft tissue infections. Despite that many efforts have been made, sensitive and reliable quantitative determination of S. aureus remains a huge challenge. Here, we depict a novel colorimetric approach for sensitive and accurate detection by combining allosteric probe-based target recognition and chain extension-based dual signal recycling. The single-strand DNA (ssDNA) products generated by the chain extension process lead to the liberation of G-quadruplex sequences, which can fold into active DNAzyme under the assistance of hemin. The active DNAzyme can work as peroxidase mimics to catalyze the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS2-)-H2O2 system, causing the color change of the system. Eventually, the method exhibits a wide detection range from 103 cfu/mL to 106 cfu/mL. The limit of detection of the approach was determined 232 cfu/mL. Considering the robust capability of the approach in S. aureus detection, we believe that it will be a potential alternative tool for biomedical research and clinical molecular diagnostics.

Hydrogel-Based Biosensors for Bacterial Infections

Hydrogels are known to have the advantages such as good biodegradability, biocompatibility, and easy functionalization, making them ideal candidates for biosensors. Hydrogel-based biosensors that respond to bacteria-induced microenvironmental changes such as pH, enzymes, antigens, etc., or directly interact with bacterial surface receptors, can be applied for early diagnosis of bacterial infections, providing information for timely treatment while avoiding antibiotic abuse. Furthermore, hydrogel biosensors capable of both bacteria diagnosis and treatment will greatly facilitate the development of point-of-care monitoring of bacterial infections. In this review, the recent advancement of hydrogel-based biosensors for bacterial infection is summarized and discussed. First, the biosensors based on pH-sensitive hydrogels, bacterial-specific secretions-sensitive hydrogels, and hydrogels directly in contact with bacterial surfaces are presented. Next, hydrogel biosensors capable of detecting bacterial infection in the early stage followed by immediate on-demand treatment are discussed. Finally, the challenges and future development of hydrogel biosensors for bacterial infections are proposed.

PNIPAM Brushes in Colloidal Photonic Crystals Enable Ex Situ Ethanol Vapor Sensing

Structural colors are formed by the periodic repetition of nanostructures in a material. Upon reversibly tuning the size or optical properties of the repetitive unit inside a nanostructured material, responsive materials can be made that change color due to external stimuli. This paper presents a simple method to obtain films of ethanol vapor-responsive structural colors based on stacked poly(N-isopropylacrylamide) (PNIPAM)-grafted silica nanoparticles. Our materials show clear, reversible color transitions in the presence of near-saturated ethanol vapor. Moreover, due to the absorption of ethanol in the PNIPAM brushes, relatively long recovery times are observed (∼30 s). Materials based on bare or poly(methyl methacrylate) (PMMA) brush-grafted silica nanoparticles also change color in the presence of ethanol vapor but possess significantly shorter recovery times (∼1 s). Atomic force microscopy reveals that the delayed recovery originates from the ability of PNIPAM brushes to swell in ethanol vapor. This renders the films highly suitable for ex situ ethanol vapor sensing.

Stimulus-responsive nonclose-packed photonic crystals: fabrications and applications

Stimulus-responsive photonic crystals (PCs) possessing unconventional nonclosely packed structures have received growing attention due to their unique capability of mimicking the active structural colors of natural organisms (for example, chameleons' mechanochromic properties). However, there is rarely any systematic review regarding the progress of nonclose-packed photonic crystals (NPCs), involving their fabrication, working mechanisms, and applications. Herein, a comprehensive review of the fundamental principles and practical fabrication strategies of one/two/three-dimensional NPCs is summarized from the perspective of designing nonclose-packed structures. Subsequently, responsive NPCs with exciting functions and working mechanisms are sorted and delineated according to their diverse responses to physical (force, temperature, magnetic, and electric fields), chemical (ions, pH, vapors, and solvents), and biological (glucose, organophosphate, creatinine, and bacteria) stimuli. We then systematically introduced and discussed the applications of NPCs in sensors, printing, anticounterfeiting, display, optical devices, etc. Finally, the current challenges and development prospects for NPCs are presented. This review not only concludes the design principle for NPCs but also provides a significant basis for the exploration of next-generation NPCs.

Smartphone-Enabled Fluorescence and Colorimetric Platform for the On-Site Detection of Hg2+ and Cl- Based on the Au/Cu/Ti3C2 Nanosheets

Smartphone-assisted fluorescence and colorimetric methods for the on-site detection of Hg²⁺ and Cl^- were established based on the oxidase-like activity of the Au-Hg alloy on the surface of Au/Cu/Ti₃C₂ NSs. The Au nanoparticles (NPs) were constructed via in-situ growth on the surface of Cu/Ti₃C₂ NSs and characterized by different characterization techniques. After the addition of Hg²⁺, the formation of Hg-Au alloys could promote the oxidization of o-phenylenediamine (OPD) to generate a new fluorescence emission peak of 2,3-diaminopenazine (ADP) at 570 nm. Therefore, a turn-on fluorescence method for the detection of Hg²⁺ was established. As the addition of Cl^- can influence the fluorescence of ADP, the fluorescence intensity was constantly quenched to achieve the continuous quantitative detection of Cl^-. Therefore, a turn-off fluorescence method for the detection of Cl^- was established. This method had good linear ranges for the detection of Hg²⁺ and Cl^- in 8.0-200.0 nM and 5.0-350.0 µM, with a detection limit of 0.8 nM and 27 nM, respectively. Depending on the color change with the detection of Hg²⁺ and Cl^-, a convenient on-site colorimetric method for an analysis of Hg²⁺ and Cl^- was achieved by using digital images combined with smartphones (color recognizers). The digital picture sensor could analyze RGB values in concentrations of Hg²⁺ or Cl^- via a smartphone app. In summary, the proposed Au/Cu/Ti₃C₂ NSs-based method provided a novel and more comprehensive application for environmental monitoring.

Advances in Portable Heavy Metal Ion Sensors

Heavy metal ions, one of the major pollutants in the environment, exhibit non-degradable and bio-chain accumulation characteristics, seriously damage the environment, and threaten human health. Traditional heavy metal ion detection methods often require complex and expensive instruments, professional operation, tedious sample preparation, high requirements for laboratory conditions, and operator professionalism, and they cannot be widely used in the field for real-time and rapid detection. Therefore, developing portable, highly sensitive, selective, and economical sensors is necessary for the detection of toxic metal ions in the field. This paper presents portable sensing based on optical and electrochemical methods for the in situ detection of trace heavy metal ions. Progress in research on portable sensor devices based on fluorescence, colorimetric, portable surface Raman enhancement, plasmon resonance, and various electrical parameter analysis principles is highlighted, and the characteristics of the detection limits, linear detection ranges, and stability of the various sensing methods are analyzed. Accordingly, this review provides a reference for the design of portable heavy metal ion sensing.

Supramolecular Photonic Hydrogel for High-Sensitivity Alkaline Phosphatase Detection via Synergistic Driving Force

Structurally-colored photonic hydrogels which are fabricated by introducing hydrogels into thin films or photonic crystal structures are promising candidates for biosensing. Generally, the design of photonic hydrogel biosensors is based on the sensor-analyte interactions induced charge variation within the hydrogel matrix, or chemically grafting binding sites onto the polymer chains, to achieve significant volume change and color variation of the photonic hydrogel. However, relatively low anti-interference capability or complicated synthesis hinder the facile and low-cost fabrication of high-performance photonic hydrogel biosensors. Here, a facilely prepared supramolecular photonic hydrogel biosensor is developed for high-sensitivity detection of alkaline phosphatase (ALP), which is an extensively considered clinical biomarker for a variety of diseases. Responding to ALP results in the broken supramolecular crosslinking and thus increased lattice distancing of the photonic hydrogel driven by synergistic repulsive force between nanoparticles embedded in photonic crystal structure and osmotic swelling pressure. The biosensor shows sensitivity of 7.3 nm spectral shift per mU mL^-1 ALP, with detection limit of 0.52 mU mL^-1 . High-accuracy colorimetric detection can be realized via a smartphone, promoting point-of-care sensing and timely diagnosis of related pathological conditions.

AIE-doped Poly(Ionic Liquid) Photonic Spheres for the Discrimination of Psychoactive Substances

Drugs of abuse has drawn intense attention due to increasing concerns to public health and safety. The construction of a sensing platform with the capability to identify them remains a big challenge because of the limitations of synthetic complexity, sensing scope and receptor extendibility. Here a kind of poly(ionic liquid) (PIL) photonic crystal spheres doped with aggregation-induced emission (AIE) luminogens was developed. As diverse noncovalent interactions involve in PIL moieties, the single sphere shows different binding affinity to a broad range of psychoactive substances. Furthermore, the dual-channel signals arising from photonic crystal structures and sensitive AIE-luminogens provide high-dimensional information for discriminative detection of targets, even for molecules with slight structural differences. More importantly, such single sphere sensing platform could be flexibly customized through ion-exchange, showing great extendibility to fabricate high-efficiency/high-throughput sensing arrays without tedious synthesis.