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The benzocoumarin dyes fluoresce negligibly in aqueous media but very strongly in cells, whereas representative conventional dyes display contrasting behaviour; the distinct emission behaviour of the fluorophores in organic solutions, in aqueous media, and in cell convinces the uniqueness of the cellular environment. The in cellulo superbright benzocoumarins also reveal an environment-insensitive emission behaviour, which is required for the reliable analysis via ratiometric imaging.

Monitoring environmental pollutants in drinking water supplies is vitally important for public health. Unfortunately, current technologies that are commonly adopted for waterborne pollutant analysis are expensive and time consuming. Surface-enhanced Raman spectroscopy (SERS) provides an inexpensive and fast way for water pollutant detection owing to the extraordinary optical properties of plasmonic nanoparticles. This book chapter is focused on label-free SERS, i.e., collecting Raman spectra directly from the targeted water pollutants. Following an overview of the principles, basic paradigms, and substrates of SERS, examples of label-free SERS detection of waterborne pollutants, including organic micropollutants, biotoxins, and pathogens, will be discussed in detail. In this book chapter, we will also talk about the challenges that label-free SERS sensors are facing for the sensitive, reproducible, and quantitative analysis of water pollutants.

Cu(II) affects our environment and living systems to various aspects, demanding its detection and quantification methods. Fluorescent detection has its own advantages, and many fluorescent sensing systems are known. As Cu(II) quenches fluorescence, a key challenge in developing fluorescent sensors is to induce fluorescence signal enhancement rather than quenching. A further challenge is to develop probes that provide ratiometric fluorescence changes at two different wavelengths. We have developed such a ratiometric sensing system for Cu(II), which is an acedan-derived dipicolylamine ligand. The probe responded to Cu(II) with a large emission wavelength shift (104 nm) from green to blue in aqueous buffer at pH 7.4; the response was specific to Cu(II) among various other metal ions. The probe was also highly sensitive (LOD = 89 nM), and thus used to detect trace amounts of Cu(II) ions in nearby river and sea water samples. Also, the probe enabled us to image intracellular Cu(II) ions through ratiometric two-photon microscopic imaging. We performed fluorescence titrations toward Cu(II) and Cu(I) at pH 7.4 and 9.0, respectively, and computational calculations on plausible metal complex intermediates, which provided insights on the sensing mechanism to propose that it involves Cu(II)-promoted enolization and subsequent reduction of Cu(II) to Cu(I). The mechanistic implication would guide us to develop other types of probes and also to re-consider the sensing mechanism involved in the known probes that involve the intramolecular charge-transfer upon metal binding.

The purpose of this study is to investigate the possibility of improving the performance of a DNA binding dye water quenching based aptasensor without changing or truncating the aptamer. To demonstrate the possibility of increasing the change in fluorescence of the aptasensor by pairing it with a suitable ssDNA probe, three ssDNA probes (probe 1, 2, and 3) were employed and the fluorescence from the bound dyes was measured. This showed that ssDNA probe 2 created the most additional binding sites. By varying the target compound concentration (0, 0.05, 0.5, 5, 50, and 500 mg L−1 4-n-nonylphenol), the corresponding change in the fluorescence signal of the unpaired and ssDNA probe paired aptasensors were measured and compared over a range of emission wavelengths. The response of all three ssDNA probe paired aptasensors showed good fit (R2 = 0.88–0.92) to a logarithmic response. The sensitivity of the aptasensor paired with ssDNA probe 2 was improved by ∼60%, whereas that of the aptasensor paired with ssDNA probe 3 was only improved by a marginal ∼3%. This study is a demonstration of using an appropriate ssDNA probe to increase the number of binding sites and hence the performance of a DNA binding dye and water quenched aptasensor. It is a possibility that can be extended to similar aptasensors without having to change or truncate the aptamer.

Endocrine-disrupting chemicals (EDCs) threaten many kinds of life throughout the world. These compounds function the same as sexual hormones, inducing precocious puberty, gynecomastia, etc., in the human body. To prevent excess exposure to nonylphenol (NP), a simple and rapid detection system is needed. In this study, we develop a nonylphenol-specific aptamer from a random single-stranded DNA library and test a rapid sensor system based on the aptamer and gold nanoparticles (AuNPs). The aptamer was screened by a methodology involving reduced graphene oxide (rGO). As a result of screening and sequencing, a DNA aptamer was developed that recognizes the target with high binding affinity (Kd = 194.2 &plusmn, 65.9 nM) and specificity. The sensor system developed using the aptamer and gold nanoparticles is sensitive (LOD = 2.239 nM). Circular dichroism (CD) spectrometry results show that the free aptamer binds to the target molecule. The aptamer was characterized using gold nanoparticles to measure UV absorbance. Our results suggest that the sensor system developed using this aptamer is useful for field diagnosis of small molecules.

Monoamine oxidases (MAOs) play an important role in Alzheimer's disease (AD) pathology. We report in vivo comonitoring of MAO activity and amyloid-beta (A beta) plaques dependent on the aging of live mice with AD, using a two-photon fluorescence probe. The probe under the catalytic action of MAO produces a dipolar fluorophore that senses A beta plaques, a general AD biomarker, enabling us to comonitor the enzyme activity and the progress of AD indicated by A beta plaques. The results show that the progress of AD has a close correlation with MAO activity, which can be categorized into three stages: slow initiation stage up to three months, an aggressive stage, and a saturation stage from nine months. Histological analysis also reveals elevation of MAO activity around A beta plaques in aged mice. The close correlation between the MAO activity and AD progress observed by in vivo monitoring for the first time prompts us to investigate the enzyme as a potential biomarker of AD.

Alzheimer’s disease (AD) is the most common form of dementia. The pathogenesis of the disease is associated with aggregated amyloid-β, hyperphosphorylated tau, a high level of metal ions, abnormal enzyme activities, and reactive astrocytes. This outlook gives an overview of fluorescent small molecules targeting AD biomarkers for ex vivo and in vivo imaging. These chemical imaging probes are categorized based on the potential biomarkers, and their pros and cons are discussed. Guidelines for designing new sensing strategies as well as the desirable properties to be pursued for AD fluorescence imaging are also provided., The molecular design strategies for small-molecule fluorescent probes detecting the potential biomarkers of Alzheimer’s disease along with their merits and limitations provide guidelines for their future developments.

The quantitative analysis of a biological analyte directly through fluorescence imaging is essential in biomedical sciences but remains as a challenge owing to the environment-sensitive nature of fluorescence intensity. We show that a fluorescent hydrogen peroxide probe based on an in cellulo super-bright benzothiazolyl-benzocoumarin dye of which emission properties are insensitive to environmental changes offers a reliable and practical means for the quantitative analysis of hydrogen peroxide in cells and tissues directly through ratiometric imaging both under one-photon and two-photon excitation conditions. Different cellular hydrogen peroxide concentrations were determined for several cell lines and also for tumour and normal tissues.

Besides the noble physical appearance of gold and silver, their novel chemical properties attracted the modern technology for various industrial, chemical and biological uses including medical applications. The widespread use of gold and silver, however, can cause potential hazards to our environment. Therefore, suitable detection methods are a prerequisite for the evaluation of their harmful effects as well as for studying their beneficial biological properties. Due to the several advantages over the conventional analytical methods, the fluorescence detection of gold and silver has become an active research area in recent years. In this review, we provide an overview of the reported fluorescent detection systems for gold and silver species, and discuss their sensing properties with promising features. The future scope of developments in this field of research is also mentioned.

8-Amino-BODIPY (boron-dipyrromethane) dyes show bright blue fluorescence. Disclosed here are synthesis and characterization of the photophysical properties of a series of functionalized 8-Amino-BODIPY (BP1-4) for protein labeling. The compact structure and solvent-insensitive absorption property of the dye are desirable features for protein labeling. For the model protein, bovine serum albumin (BSA), the labeling proceeds under mild condition via amide bond formation or thiol-ene conjugation with maintaining the bright blue fluorescence. The chromatography and mass spectroscopy analysis clearly support the labeling of the BODIPY dye on the BSA. The protein labeling with blue-emitting BODIPY would be applicable for studying protein dynamics and fluorescence resonance energy transfer (FRET) with intrinsic biomolecules.