Real-time PMIRRAS studies of in situ growth of C11Eg6OMe on gold and immersion effects

The Effect of Chemical Structure of OEG Ligand Shells with Quaternary Ammonium Moiety on the Colloidal Stabilization, Cellular Uptake and Photothermal Stability of Gold Nanorods

PURPOSE

Plasmonic photothermal cancer therapy by gold nanorods (GNRs) emerges as a promising tool for cancer treatment. The goal of this study was to design cationic oligoethylene glycol (OEG) compounds varying in hydrophobicity and molecular electrostatic potential as ligand shells of GNRs. Three series of ligands with different length of OEG chain (ethylene glycol units = 3, 4, 5) and variants of quaternary ammonium salts (QAS) as terminal functional group were synthesized and compared to a prototypical quaternary ammonium ligand with alkyl chain - (16-mercaptohexadecyl)trimethylammonium bromide (MTAB).

METHODS

Step-by-step research approach starting with the preparation of compounds characterized by NMR and HRMS spectra, GNRs ligand exchange evaluation through characterization of cytotoxicity and GNRs cellular uptake was used. A method quantifying the reshaping of GNRs was applied to determine the effect of ligand structure on the heat transport from GNRs under fs-laser irradiation.

RESULTS

Fourteen out of 18 synthesized OEG compounds successfully stabilized GNRs in the water. The colloidal stability of prepared GNRs in the cell culture medium decreased with the number of OEG units. In contrast, the cellular uptake of OEG+GNRs by HeLa cells increased with the length of OEG chain while the structure of the QAS group showed a minor role. Compared to MTAB, more hydrophilic OEG compounds exhibited nearly two order of magnitude lower cytotoxicity in free state and provided efficient cellular uptake of GNRs close to the level of MTAB. Regarding photothermal properties, OEG compounds evoked the photothermal reshaping of GNRs at lower peak fluence (14.8 mJ/cm2) of femtosecond laser irradiation than the alkanethiol MTAB.

CONCLUSION

OEG+GNRs appear to be optimal for clinical applications with systemic administration of NPs not-requiring irradiation at high laser intensity such as drug delivery and photothermal therapy inducing apoptosis.

Controlled DNA Patterning by Chemical Lift-Off Lithography: Matrix Matters

Nucleotide arrays require controlled surface densities and minimal nucleotide-substrate interactions to enable highly specific and efficient recognition by corresponding targets. We investigated chemical lift-off lithography with hydroxyl- and oligo(ethylene glycol)-terminated alkanethiol self-assembled monolayers as a means to produce substrates optimized for tethered DNA insertion into post-lift-off regions. Residual alkanethiols in the patterned regions after lift-off lithography enabled the formation of patterned DNA monolayers that favored hybridization with target DNA. Nucleotide densities were tunable by altering surface chemistries and alkanethiol ratios prior to lift-off. Lithography-induced conformational changes in oligo(ethylene glycol)-terminated monolayers hindered nucleotide insertion but could be used to advantage via mixed monolayers or double-lift-off lithography. Compared to thiolated DNA self-assembly alone or with alkanethiol backfilling, preparation of functional nucleotide arrays by chemical lift-off lithography enables superior hybridization efficiency and tunability.

Helical versus all-trans conformations of oligo(ethylene glycol)-terminated alkanethiol self-assembled monolayers

The complex mixture of conformational states exhibited by oligo(ethylene glycol)-terminated alkanethiols on Ag and Au surfaces is explored by polarization-dependent X-ray absorption spectroscopy. Three self-assembled monolayers (SAMs) with known helical or all-trans conformations are used as references to characterize a SAM with unknown conformations. This case study is used as a prototype for developing a systematic framework to extract the conformations of SAMs from the polarization dependence of several orbitals. In the case at hand, these are associated with the C-H/Rydberg bonds of the alkane, the C-H/Rydberg bonds of ethylene glycol, and the C-C bonds of the backbone. The C-H/Rydberg orbitals of the alkane and ethylene glycol are distinguished via the chemical shift of the corresponding C 1s core levels.

Comparison of the influence of humidity and D-mannitol on the organization of tetraethylene glycol-terminated self-assembled monolayers and immobilized antimicrobial peptides

We report the use of polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) to characterize the effects of relative humidity (RH) and d-mannitol on the conformations of tetraethylene glycol (EG4)-terminated self-assembled monolayers (SAMs) and immobilized antimicrobial peptides (Cecropin P1 and a hybrid of Cecropin A (1-8) and Melittin (1-18)). These results are used to assess the extent to which d-mannitol can substitute for water in promoting conformational states of the SAMs and oligopeptides similar to those induced by hydration. Our measurements reveal a red shift of the COC asymmetric stretching vibration of the EG4-terminated SAMs with increasing humidity, consistent with a transition from a mixed all-trans/helical (7/2 helix) conformation at 0% RH to a predominantly helical conformation at 90% RH. Significantly, under dry conditions, a thin (2 nm in thickness) overlayer of d-mannitol generated the COC spectroscopic signature of the EG4-terminated SAM measured at high humidity. Comparisons of the effects of humidity and d-mannitol on the secondary structure of the two oligopeptides also revealed both to cause the amide I peak positions, which were measured in dry air (and without d-mannitol) to correspond to α-helical conformations, to undergo red-shifts. The magnitudes of the red-shifts, however, were more pronounced for dry d-mannitol than for high RH, with Cecropin P1 and the hybrid peptide exhibiting amide I peak positions under d-mannitol consistent with bulk aqueous solution secondary structures (random and β-sheet, respectively). These results are discussed in the context of prior reports of the tendency of d-mannitol to form glassy states in the absence of water. Overall, the results presented in this paper support the hypothesis that d-mannitol can substitute, in at least some ways, for the influence of water on the conformational states of biologically relevant molecules at interfaces. The results provide guidance for the design of interfaces for water-free biologics.

Gold nanoparticles decorated with oligo(ethylene glycol) thiols: surface charges and interactions with proteins in solution

We have studied oligo(ethylene glycol) (OEG) thiol self-assembled monolayer (SAM) coated gold nanoparticles (AuOEG) and their interactions with proteins in solutions using electrophoretic and dynamic light scattering (ELS and DLS). The results are compared with poly(ethylene glycol) (PEG) thiol coated AuNPs (AuPEG). We show that both AuOEG and AuPEG particles carry a low net negative charge and are very stable (remaining so for more than one year), but long-term aging or dialysis can reduce the stability. If the decorated AuNPs are mixed with bovine serum albumin (BSA), both effective size and zeta-potential of the AuNPs remain unchanged, indicating no adsorption of BSA to the colloid surface. However, when mixed with lysozyme, zeta-potential values increase with protein concentrations and lead to a charge inversion, indicating adsorption of lysozyme to the colloid surface. The colloidal solutions of AuOEG become unstable near zero charge, indicated by a cluster peak in the DLS measurements. The AuPEG solutions show similar charge inversion upon addition of lysozyme, but the solutions are stable under all experimental conditions, presumably because of the strong steric effect of PEG. Washing the protein bound colloids by centrifugation can remove only part of the adsorbed lysozyme molecules indicating that a few proteins adsorb strongly to the colloids. The effective charge inversion and rather strongly bound lysozyme on the colloid surface may suggest that in addition to the charges formed at the SAM-water interface, there are defects on the surface of the colloid, which are accessible to the proteins. The results of this study of surface charge, and stability shed light on the interaction with proteins of SAM coated AuNPs and their applications.

On the stability of oligo(ethylene glycol) (C11EG6OMe) SAMs on gold: behavior at elevated temperature in contact with water

In this study the temperature dependent confor-mation of hexa(ethylene glycol) self-assembling monolayers (SAMs) under aqueous conditions (in situ) is investigated. To this end characteristic absorption modes in the fingerprint region (1050-1500 cm(-1)) were monitored with real-time polarization modulation infrared spectroscopy. We found a temperature induced conformational change from predominantly helical to helical/all-trans. The process may be divided into two temperature regimes. Up to 40 °C the process is reversible after drying the monolayers in air and successive reimmersion in water, indicating a strong binding of the water molecules to the SAM. At higher temperatures, the conformational change is irreversible. Additionally, a rapid change to a larger mode width and a shift of the mode position to higher wavenumbers (blue-shift) at about 50 °C indicates structural changes caused by decreasing crystallinity of the SAM. While the conformational changes up to 40 °C are supposed to originate from an increased conformational freedom in combination with a stronger interaction with water molecules, the irreversibility and rapid change of mode characteristics at higher temperatures indicate chemical degradation. Complementary measurements in air show a fast and virtually complete reversibility up to 40 °C underlining the effect of the interaction of the ethylene glycol moiety with water. At temperatures above 50 °C modes indicating ester and formate groups appear, supporting the idea of chemical degeneration. Moreover, the temperature behavior is coverage dependent. At incomplete coverage the structural order of the SAM starts decreasing at lower temperatures. This study shows, that the conformational and structural change of hexa(ethylene glycol) SAMs at elevated temperature is an interplay of conformational changes of the SAM, its interaction with water and at higher temperatures its chemical degradation. Our experiments also underline the importance of the in situ analysis on the film structure.