Analytische Chemie
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The development of a Lab-on-a-Chip (LoC) is presented, which can detect reactive phosphorous compounds in the gas phase in combination with an optochemical hand-held sensor. The LoC prototype contains three pairs of sensing materials containing fluorescent indicator dyes in various carrier materials. By measuring the fluorescence response to phosphoryl chloride, a surrogate compound, the detection of chemical warfare agents (CWAs) in gas phase becomes possible within seconds, introducing a novel approach to CWA detection.
PFAS Sensors
(2023)
This contribution provides an introduction to the development of sensors for PFAS analysis, presents the most common approaches, and describes the opto-microfluidic strategy in combination with polymerizable indicators and detection matrices currently being pursued by the Chemical and Optical Sensing Division at BAM.
Per- and polyfluoroalkyl substances (PFAS) are a class of man-made organo-fluorine chemicals that have become environmental contaminants of emerging concern, originating from a variety of materials such as adhesive, stain- and oil-resistant coatings, firefighting foams, etc. The high strength of this C-F bond makes PFAS thermodynamically stable and resistant to (bio)degradation, thus retaining them in the environment over time. Perfluoroalkyl carboxylic acids (PFCAs), one category of the most used PFAS, consist of a fully fluorinated carbon backbone and a charged carboxylic acid headgroup, and have been classified as Substances of Very High Concern (SVHC) and added to the REACH Candidate List due to their persistence in the environment, non-biodegradability and toxicological effects.[1-2] Traditional techniques for the analysis of PFCAs include GC-MS, HRMS and HPLC-based approaches, which are laborious, not portable, costly and require trained personnel. In contrast, fluorescence assays can be designed as easy-to-operate, portable and cost-effective methods with high sensitivity and fast response. Integration of fluorescent probes with an adequately miniaturized assay enables a promising alternative for PFCAs analysis.
Here, a novel guanidine fluorescent probe has been synthesized and fully characterized for the detection of PFCAs in a biphasic extract-&-detect assay. The fluorescent probe was then incorporated into polymeric matrices supported by a red dye-doped SiO2 nanoparticle to construct a dual-emission sensing platform. Such a system allows precise and selective detection of PFCAs, reducing the interference of competitors, matrix effects and other factors except for the PFCAs. The system was then employed in a droplet-based microfluidic setup which offers a portable and easy to operate detection platform.
The reversible analyte-induced switching between a colourless leuco form of a dye and its coloured all-π-conjugated form is one of the oldest concepts in probe- or indicator-based optical analysis,1 constituting so-called “one-color indicators”.2 In contrast to colour changes in the visible region, for which usually a bond-forming or a non-covalent interaction between a functional group on an indicator and an analyte is responsible, the extraordinarily large shifts of 150–250 nm seen for leuco-to-all-π-conjugated transformation are only possible when the reaction takes place directly at a site that is an intrinsic part of a dye’s π-system; the classic case are triphenylmethane dyes.3 Despite its obvious potential, this approach has not been a very popular area of scientific research for decades, perhaps because it is much more difficult to implement selectivity than by modifying terminal functional groups. However, in recent years, reaction-based fluorescence probes have attracted increasing attention by virtue of their superior sensitivity.4,5
Nowadays, rhodamines and fluoresceins are by far the most prominent groups of reaction-based indicators.5 They are usually switched between a colourless and non-fluorescent and a greenish-yellowish absorbing and yellowish-reddish emitting form. Typically, the coloured and fluorescent form is ionic, while the colourless form is neutral, reactions having mainly be designed to occur at the end groups of the xanthenoid π-system. BODIPY dyes, with their favourable spectroscopic and chemical properties as well as facile wavelength tunability features have only very recently been explored into this direction.6,7 Reaction at the core dipyrrin framework of these dyes led to strong colour and fluorescence modulations with potential applications in materials sciences6 and cell imaging.7 The present contribution will highlight the mechanisms at play and the sensing performance realized so far, and will compare core-reactive approaches to reaction-based signalling that involves π-extension of BODIPYs.8,9
Chlorination of pool water and wastewater, in food and pharmaceutical production, as well as in pesticide and paper manufacturing is a routinely used technique. However, the amount of chlorine in water must be strictly adjusted, to ensure enough concentration to kill pathogenic bacteria and viruses, while preventing too high concentrations inducing negative effects on human health. As an indicator, a molecular fluorescent probe based on a BODIPY structure was designed. This indicator exhibits a sensitive and selective fluorescence response upon increasing concentrations of hypochlorite in aqueous solvent mixtures. Real-time analyses became possible after the integration of this fluorescent indicator into newly designed 2D & 3D microfluidic chips incorporating a passive sinusoidal mixer and a micro-hydrocyclone, respectively. A comparison of the two microfluidic systems, including their ability to prevent accumulation or circulation of microbubbles, has shown excellent fluidic behaviour for the micro-hydrocyclone device. This system was distinctly more robust against gas bubbles, showed a higher signal gain and allowed to halve the limit of detection to 0.02 mg L–1. The use of the 3D system to quantify the chlorine content of pool water samples for sensitive and quantitative chlorine monitoring has been demonstrated.
Supramolecular chemistry, fluorescence detection, hybrid (nano)materials and device miniaturization are in themselves highly interesting areas of research, yet especially their combination paves the way to (bio)chemical analysis systems that show outstanding performance. The lecture gives an overview of the toolbox of single components developed in BAM’s Chemical and Optical Sensing Division over the years, and how their combination can result in powerful sensors, quick tests and assays. While at the core of a development is the analytical problem, that is, the determination of a certain analyte in a sample of interest with the required sensitivity and selectivity by a specific end user in a given setting, signaling mechanisms, recognition elements, signal transduction modes, materials functionalization, device design and system integration are adequately chosen, tailored and adapted. Examples including molecularly imprinted polymers, hybrid mesoporous nanomaterials, gated indicator release systems, microfluidic devices, test strips and smartphone-based analysis will be presented.
Molecularly Imprinted Polymers with Integrated Fluorescence as Versatile Biomimetic Sensing Matrices
(2018)
Molecularly imprinted polymers (MIPs) are an established, versatile and high-performance matrix for the selective separation or enrichment of (bio)chemical species, especially small molecules of biochemical or environmental relevance. MIPs are prepared through the polymerization of a mixture of functional monomers and cross-linkers in the presence of the template with subsequent extraction of the latter. Conceptionally, this process can be seen as mimicking in a strongly accelerated, though single-step manner a biological process such as antibody formation. Because the resulting MIPs contain cavities in their matrix that are complementary in size, shape and electronic/ electrostatic or hydrogen bonding demand to the imprinted target molecule or template, these polymers are frequently termed “artificial antibodies”. Compared to natural antibodies, they are chemically and physically much more robust. Regarding sensitivity and selectivity, however, there is still a gap to bridge before MIPs can fully compete with antibodies.
Another favorable aspect that distinguishes MIPs from antibodies is that they can be endowed with an explicit function, allowing the use of MIPs in applications that require more than only an efficient binder. For instance, if specifically designed and polymerizable fluorescent indicators are integrated as functional monomers into a MIP, direct fluorescence sensing can be accomplished. Because MIPs can be prepared in a variety of different formats, their combination with miniaturized or other specific analytical techniques or sensory devices is possible, especially when the transduction mode is light. This presentation will introduce basic design considerations, challenges, limitations and the potential that lies with such sensor materials with some recent examples of our group, targeting various organic oxoanions as analytes.
Molecularly imprinted polymers (MIPs) are an established and powerful matrix for the selective enrichment and separation of chemical species, especially of small organic molecules. Because MIPs contain cavities in their matrix that are complementary in size, shape and electronic/electrostatic or hydrogen bonding demand to the imprinted target molecule or template, they are frequently termed “artificial antibodies”. Compared to natural antibodies, they are chemically and physically much more robust. MIP formation proceeds through the polymerization of a mixture of functional monomers and cross-linkers in the presence of the template with subsequent extraction of the latter. While this strategy has been successfully employed for separation MIPs since decades, the development of sensory MIPs has long been limited to the combination of a MIP as enrichment phase with a separate signalling element in a discontinuous fashion, being unsuitable for many sensing applications. Only very recently, the implementation of specifically designed fluorescent probes into MIPs has been successfully accomplished, fluorescence being one of the most versatile, sensitive and easily miniaturizable techniques. This presentation will introduce basic design considerations, challenges, limitations and the potential that lies with such sensor materials on the background of the work carried out in our group.
Integrating fluorescent probes with sensing matrices presents a major challenge because usually, when confined in a rather rigid matrix, fluorophores tend to behave completely different than for instance in the molecular state in solution. The lecture reviews the major strategies that have been devised recently to circumvent such issues with special focus on the works carried out in this field by BAM’s Chemical and Optical Sensing Division. Moreover, it will be shown that by using certain strategies not only can the response behavior be retained but synergistic effects can even endow the hybrid with a much better performance than the probe molecule alone.
Macrocycle-containing fluorescent probes continue to be one of the most popular classes of indicator molecules for the sensitive optical detection of ionic inorganic analytes, in particular metal ions, since the first integration of crown ether building blocks into chromophoric π systems more than 30 years ago. However, whereas a large multitude of such probes have been described for operation in organic or mixed aqueous environments, the step to realistic analytical media such as water samples, aqueous food extracts or body fluids is still scarce. On one hand, this is due to considerably low complex stability constants for instance for alkali and alkaline-earth metal ions. On the other hand, many classes of organic dyes that show favourable spectroscopic properties and ion-induced responses in organic solvents are not soluble in neat aqueous media and/or exhibit only significantly quenched fluorescence, even if transition metal ions are binding strongly to the receptor unit. A way to circumvent such problems in a rather simple manner is the steric incorporation of fluorescent probes into mesoporous silica nanomaterials. The local polarity in such pores, whether nascent or modified appropriately with functional silanes, resembles much more a quasi-organic environment while at the same time allowing for free diffusion of water and its cargo.
Two examples of powerful hybrid signalling systems will be presented, utilizing charge transfer-type indicator molecules that show inferior sensing properties in their molecular state. One example discusses mercury(II) determination over an extended concentration range and the other discusses silver(I) and mercury(II) discrimination simply on the basis of photophysical effects retained in the hybrid materials.