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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.
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.
N-Benzyl aroyl-S,N-ketene acetals can be readily synthesized by condensation of aroyl chlorides and N-Benzyl 2-methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid-state emission and aggregation-induced emission characteristics.
Varying the substituent from electron-donating to electronwithdrawing enables the tuning of the solid-state emission Color from deep blue to red.
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.
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 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.
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.
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.
Carboxy, amino, and thiol groups play a critical role in a variety of physiological and biological processes and are frequently used for bioconjugation reactions. Moreover, they enable size control and tuning of the surface during the synthesis of particle systems. Especially, thiols have a high binding affinity to noble metals and semiconductors (SC). Thus, simple, inexpensive, robust, and fast methods for the quantification of surface groups and the monitoring of reactions involving ligands are of considerable importance for the characterization of modified or stabilized nanomaterials including polymers.
We studied the potential of the Ellman’s assay, recently used for the quantification of thiol ligands on SC nanocrystals by us1 and the 4-aldrithiol assay for the determination of thiol groups in molecular systems and on polymeric, noble and SC nanomaterials. The results were validated with ICP-OES and reaction mechanisms of both methods were studied photometrically and with ESI-TOF-MS.
The investigation of the reaction mechanisms of both methods revealed the influence of different thiols on the stoichiometry of the reactions2, yielding different mixed disulfides and the thiol-specific products spectroscopically detected. The used methods can quantify freely accessible surface groups on nanoparticles, e.g., modified polystyrene nanoparticles. For thiol ligands coordinatively bound to surface atoms of, e.g., noble or SC nanomaterials, depending on the strength of the thiol-surface bonds, particle dissolution prior to assay performance can be necessary.
We could demonstrate the reliability of the Ellman’s and aldrithiol assay for the quantification of surface groups on nanomaterials by ICP-OES and derived assay-specific requirements and limitations. Generally, it is strongly recommended to carefully control assay performance for new samples, components, and sample ingredients to timely identify possible interferences distorting quantification.
Many applications of nanometer- and micrometer-sized particles include their surface functionalization with linkers, sensor molecules, and analyte recognition moieties like (bio)ligands. This requires knowledge of the chemical nature and number of surface groups accessible for subsequent coupling reactions. Particularly attractive for the quantification of these groups are spectrophotometric and fluorometric assays, which can be read out with simple instrumentation. In this respect, we present here a novel Family of cleavable spectrophotometric and multimodal reporters for conjugatable amino and carboxyl surface groups on nano- and microparticles. This allows determination of particle-bound labels, unbound reporters in the supernatant, and reporters cleaved off from the particle surface, as well as the remaining thiol groups on particle, by spectrophotometry and inductively coupled optical emission spectrometry (32S ICP-OES). Comparison of the performance of these cleavable reporters with conductometry and conventional labels, utilizing changes in intensity or Color of absorption or emission, underlines the analytical potential of this versatile concept which elegantly circumvents Signal distortions by scattering and Encoding dyes and enables straightforward validation by method comparison.