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Risk assessment of nanomaterials requires not only standardized toxicity studies but also validated methods for nanomaterial surface characterization with known uncertainties. In this context, a first bilateral interlaboratory comparison on Surface group quantification of nanomaterials is presented that assesses different reporter-free and labeling methods for the quantification of the total and accessible number of amine functionalities on commercially available silica nanoparticles that are widely used in the life sciences. The overall goal of this comparison is the identification of optimum methods as well as achievable measurement uncertainties and the comparability of the results across laboratories. We also examined the robustness and ease of implementation of the applied analytical methods and discussed method-inherent limitations. In summary, this comparison presents a first step toward the eventually required standardization of methods for surface group quantification.
Molecular aggregation alters the optical properties of a system as fluorescence may be activated or quenched. This is usually described within the well-established framework of H- and J-aggregates. While H-aggregates show nonfluorescent blueshifted absorption bands with respect to the isolated monomer, Jaggregates are fluorescent displaying a redshifted peak. In this publication, we employ a combined approach of experiment and theory to study the complex aggregation features and photophysical properties of diaminodicyanoquinone derivatives, which show unusual and puzzling nonfluorescent redshifted Absorption bands upon aggregation. Our theoretical analysis demonstrates that stable aggregates do not account for the experimental observations.
Instead, we propose an unprecedented mechanism involving metastable dimeric species formed from stable dimers to generate nonfluorescent J-aggregates. These results represent a novel kind of aggregation-induced optical effect and may have Broad implications for the photophysics of dye aggregates.
Merocyanine–triarylamine bichromophores are readily synthesized by sequentially Pd-catalyzed insertion alkynylation–Michael–Suzuki four-component reactions. White-light emissive systems form upon aggregation in 1 : 99 and 0.1 : 99.9 vol% CH2Cl2–cyclohexane mixtures, ascribed to aggregation-induced dual emission (AIDE) in combination with partial energy transfer between both chromophore units as supported by spectroscopic studies.
We present a comparative study of the spectroscopic properties of the donor–acceptor–donor substituted dyes triphenylamine-allylidenemalononitrile-julolidine (TMJ) and triphenylamine-allylidenemalononitriletriphenylamine (TMT), bearing one and two propeller-like triphenylamine donor moieties, in solvents of varying polarity and viscosity and in the aggregated and solid state. Our results reveal control of the aggregation-induced spectroscopic changes and the packing motifs of the dye molecules in the solid state by the chemical nature and structure of the second nitrogen-containing donor, i.e., a planar and a rigid julolidine or a twisted triphenyl group. Assuming that the TMT and TMJ aggregates show a comparable arrangement of the molecules to the respective crystals, these different molecular interactions in the solid state are responsible for aggregation induced emission (AIE) in the case of TMT and its absence for TMJ. Moreover, a versatile strategy for the fluorescence enhancement of only weakly emissive AIE dyes is shown, turning these dyes into bright nanoscale fluorescent reporters by using them as stains for preformed polymer particles.
Synthesis and Optical Quantification of Surface Groups on Organic and Inorganic Particle-Carriers
(2020)
Differently sized organic and inorganic particles are of great interest in the life and material sciences, as they can be used e.g. as drug carriers, fluorescent sensors, and multimodal labels in bioanalytical assays and imaging applications.1 Particle performance in such applications depends mainly on the sum of their intrinsic physicochemical properties. Here, the surface chemistry, i.e., the total number of surface functional groups (FG) and the number of FG accessible for subsequent modification with ligands and/or biomolecules, is one of the key parameters. Moreover, the surface chemistry of these materials controls the behavior and fate of the particles when released to the environment or taken up by cells. Nevertheless, it is still relatively rare that FG are quantified in particle safety studies. Methods for FG quantification should be simple, robust, reliable, fast, and inexpensive, and allow for the characterization of a broad variety of materials differing in size, chemical composition, and optical properties.
Aiming at the development of simple, versatile, and multimodal tools for the quantification of bioanalytically relevant FG such as amine2,3, carboxy2,3, thiol, and aldehyde4 functionalities, we designed a catch-and-release assay utilizing cleavable probes that enable the quantification of the cleaved-off reporters in the supernatant after particle separation, and thus, circumvent interferences resulting from particle light scattering and sample-inherent absorption or emission.2 The potential of our cleavable probes for the quantification of carboxy and amino groups was demonstrated for commercial and custom-made polymer and silica particles of varying FG densities, underlining the benefit of the catch-and-release assays as a versatile method for the FG quantification on all types of transparent, scattering, absorbing and/or fluorescent particles.2,3 In the future, our cleavable probe strategy can be easily adapted to other analytical techniques requiring different reporters, or to different types of linkers that can be cleaved thermally, photochemically, or by pH, utilizing well-established chemistry, e.g. from drug delivery systems. It can contribute to establish multi-method characterization strategies for particles to provide a more detailed picture of the structure-properties relationship and thus can support the design of sustainable and safe(r) materials.
Two sets of polystyrene nanoparticles (PSNPs) with comparable core sizes but different carboxyl group densities were made and separated using asymmetric flow field flow fractionation (AF4), capillary electrophoresis (CE), and the off-line hyphenation of both methods. Our results revealed the significant potential of two-dimensional off-line AF4-CE hyphenation to improve the separation and demonstrated for the first time, the applicability of CE to determine the functional group density of nanoparticles (NPs). Compared to the result acquired with conductometric titration, the result obtained with synthesized 100 nm sized PSNPs revealed only a slight deviation of 1.7%. Commercial 100 nm sized PSNPs yielded a deviation of 4.6 %. For 60 nm sized PSNPs, a larger deviation of 10.6 % between both methods was observed, which is attributed to the lower separation resolution.
Aldehyde moieties on 2D-supports or microand nanoparticles can function as anchor groups for the attachment of biomolecules or as reversible binding sites for proteins on cell surfaces. The use of aldehyde-based materials in bioanalytical and medical settings calls for reliable methods to detect and quantify this functionality. We report here on a versatile concept to quantify the accessible aldehyde moieties on particle surfaces through the specific binding and subsequent release of small reporter molecules such as fluorescent dyes and nonfluorescent chromophores utilizing acylhydrazone formation as a reversible covalent labeling strategy. This is representatively demonstrated for a set of polymer microparticles with different aldehyde labeling densities. Excess reporter molecules can be easily removed by washing, eliminating inaccuracies caused by unspecific adsorption to hydrophobic surfaces. Cleavage of hydrazones at acidic pH assisted by a carbonyl trap releases the fluorescent reporters rapidly and quasi-quantitatively and allows for their fluorometric detection at low concentration. Importantly, this strategy separates the signal-generating molecules from the bead surface. This circumvents common issues associated with light scattering and signal distortions that are caused by binding-induced changes in reporter fluorescence as well as quenching dye−
dye interactions on crowded particle surfaces. In addition, we demonstrate that the release of a nonfluorescent chromophore via disulfide cleavage and subsequent quantification by absorption spectroscopy gives comparable results, verifying that both assays
are capable of rapid and sensitive quantification of aldehydes on microbead surfaces. These strategies enable a quantitative
comparison of bead batches with different functionalization densities, and a qualitative prediction of their coupling efficiencies in bioconjugations, as demonstrated in reductive amination reactions with Streptavidin.
Quantification of Aldehydes via Catch and Release of Reporter Chromophores on Polymeric Microbeads
(2019)
Aldehyde-functionalized materials have found widespread use in bioconjugation applications. For example, coupling of aldehyde surface groups with proteins, peptides or amine-functionalized oligonucleotides can readily produce biomolecule-decorated chip and bead surfaces for multiplex analyses. Furthermore, aldehyde-modified nanoparticles can possess bioadhesive properties that can prolong their retention time in biological compartments. These emerging novel bioanalytical and biomedical applications call for reliable tools and methods to detect and quantify accessible aldehyde functionalities on the surface of 2D- and 3D-supports.
We present here a versatile concept to quantify the amount of accessible aldehyde moieties on the surface of PMMA particles through the specific binding and subsequent release of small reporter molecules such as absorbing and fluorescent dyes utilizing acylhydrazone formation as a reversible covalent labeling strategy. Unbound reporter molecules can be easily removed by washing steps, eliminating inaccuracies caused by unspecific adsorption to hydrophobic surfaces. Cleavage of the hydrazones at acidic pH assisted by a carbonyl trap releases the optical reporters rapidly and quantitatively and allows for their optical detection at low concentration. Importantly, this strategy separates the signal-generating molecules from the bead surface, thereby circumventing common pitfalls of optical assays associated with light scattering and signal distortions due to binding-induced changes in reporter fluorescence and quenching dye-dye interactions on crowded material surfaces. The potential of this catch-and-release strategy for surface group quantification is representatively demonstrated for a set of microparticles functionalized with different aldehyde densities. This concept is validated by a colorimetric assay with a different optical probe, which contains a reductively cleavable disulfide bond and a reporter that can be quantified photometrically in solution after its release. The excellent match of the results of both optical assays confirms their suitability for the rapid and sensitive quantification of aldehydes on microbead surfaces. These simple catch-and-release assays are excellent tools for process control during bead fabrication and the comparison of different bead batches. Their potential for predicting biomolecule coupling efficiencies in bioconjugation reactions is currently assessed in reductive amination reactions with streptavidin.
Aldehyde-functionalized materials have found broad use in bioconjugation applications. For example, coupling of aldehyde surface groups with to proteins or amine-functionalized oligonucleotides can readily produce biomolecule-covered chip microarray and bead surfaces for multiplex analyses. Additionally, aldehyde-modified nanoparticles can possess bioadhesive properties that can extend their retention time in biological compartments. These emerging novel bioanalytical applications call for reliable tools and methods to detect and quantify accessible aldehyde functionalities.
We present here a straightforward concept to quantify the amount of accessible aldehyde moieties on the surface of polymethylmethacrylate (PMMA) particles through the specific binding and subsequent release of small reporter molecules such as absorbing and fluorescent dyes utilizing hydrazone formation as a reversible covalent labeling strategy. Unbound reporter molecules can be easily removed by washing steps, eliminating inaccuracies caused by unspecific adsorption to hydrophobic surfaces. Cleavage of the hydrazones at acidic pH assisted by a carbonyl trap releases the optical reporters rapidly and quantitatively and allows for their optical detection at low concentration. Importantly, this strategy separates the signal-generating molecules from the bead surface, thereby circumventing light scattering and signal distortions due to binding-induced changes in reporter fluorescence and quenching dye-dye interactions on crowded material surfaces. The potential of this catch-and-release strategy for surface group quantification is representatively demonstrated for a set of microparticles functionalized with different aldehyde densities. This concept is validated by a colorimetric assay with a different optical probe, which contains a reductively cleavable disulfide bond and a reporter that can be quantified photometrically in solution after its cleavage. The excellent match of the results of both optical assays confirms their suitability for the rapid and sensitive quantification of aldehydes on microbead surfaces. These simple catch-and-release assays are excellent tools for process control during bead fabrication and the comparison of different bead batches.
We designed and synthesized triphenylamine based and coumarin fused rhodamine hybrid dyes and characterized using 1H, 13C NMR and HR-LCMS analysis. Both the newly synthesized hybrid dyes were found to show red shifted absorption as well as emissions and large Stokes shift (40e68 nm) as compared to the small Stokes shift (25e30 nm) of reported dyes Rhodamine B and 101. Photophysical properties of these dyes were studied in different solvents and according to the solvents acidity or basicity they preferred to remain in their spirocyclic or open form in different ratio. We studied the spirocyclic as well as open form derivatives of these dyes for their viscosity sensitivity in three different mixture of solvents i.e. polar-protic [EtOH-PEG 400], polar-aprotic [toluene-PEG 400] and non-polaraprotic [toluene-paraffin]. They are found to show very high viscosity sensitivity in polar-protic mixture of solvents [EtOH-PEG 400] and hence concluded that both polarity as well as viscosity factor worked together for the higher emission enhancement rather than only viscosity factor. As these dyes showed very high viscosity sensitivity in their spirocyclic as well as open form, they can be utilized as viscosity sensors in visible as well as deep red region. We also correlated our experimental finding theoretically by using Density Functional theory computations.