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 (S 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.
Aggregation-induced
emission (AIE) has been meanwhile observed
for many dye classes and particularly for fluorophores containing
propeller-like groups. Herein, we report on the AIE characteristics
of a series of four hydrophobic pyrrolidinylvinylquinoxaline (PVQ)
derivatives with phenyl, pyrrolyl, indolyl, and methoxythienyl substituents
used to systematically vary the torsion angle between this substituent
at the quinoxaline C2 position and the planar PVQ moiety. These molecules,
which are accessible via four- or five-component one-pot syntheses,
were spectroscopically studied in organic solvents and solvent–water
mixtures, as dye aggregates, solids, and entrapped in polystyrene
particles (PSP). Steady-state and time-resolved fluorescence measurements
revealed a strong fluorescence enhancement for all dyes in ethanol–water
mixtures of high water content, accompanying the formation of dye
aggregates with sizes of a few hundred nm, overcoming polarity and
H-bonding-induced fluorescence quenching of the charge-transfer-type
emission of these PVQ dyes. The size and shape of these dye aggregates
and the size of the AIE effect are controlled by the water content
and the substituent-dependent torsion angle that influences the nucleation
process and the packing of the molecules during aggregation. Staining
of 1 μm-sized carboxy-functionalized PSP with the PVQ dyes resulted
also in a considerable increase in the fluorescence quantum yield
and lifetime, reflecting the combined influence of the restricted
molecular motion and the reduced polarity of the dye microenvironment.
A comprehensive systematic study of absorption and fluorescence properties in solvents of varying viscosity and polarity of three novel and red-emitting coumarin-rhodamine hybrid derivatives with differences in the rigidity of their substituents is presented. This includes ethanol-polyethylene glycol, toluene-polyethylene glycol, and toluene-paraffin mixtures. Moreover, protonation-induced effects on the spectroscopic properties are studied. A viscosity-induced emission enhancement was observed for all coumarin-rhodamine hybrid derivatives. MCR2 bearing a julolidine donor showed the expected low sensitivity to viscosity whereas MCR3 with its freely rotatable diphenylamino substituent revealed a particularly pronounced sensitivity to this parameter. Moreover, MCR2 shows an enhancement in emission in the open, i.e., protonated form in conjunction with a largely Stokes shift fluorescence in the deep red spectral region. This enables the application of these dyes as viscosity sensors and as far red emitting pH-sensitive probes.
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