Here, we describe the stability of solutions of various Pd and PdAg organic-protected nanoparticles (NPs) in the presence of H2 and their selectivity and reactivity as catalysts for hydrogenation or isomerization of allyl alcohol. Pd and Pd91Ag9 NPs stabilized with hexadecylamine (C16NH2) ligands are stable against H2-induced aggregation, whereas those stabilized with octylamines (C8NH2) and dodecylamines (C12NH2) precipitate within 1 h. The stability of C16NH2 Pd NPs is comparable to that of hexanethiolate (C6S)-protected Pd NPs and mixed monolayer C6S/C8NH2 (1/1) Pd NPs that were studied previously. The stability of C16NH2 Pd NPs decreases as the alkylamine/PdII ratio used in the synthesis decreases from 12:1 to 6:1 to 3:1. A bilayer or partial bilayer of C16NH2 ligands forms around the Pd core for ratios greater than 6:1, which explains the higher stability of these NPs against aggregation. The various Pd and PdAg NPs catalyzed the hydrogenation and isomerization of allyl alcohol in the presence of H2 with various selectivities and reactivities. C6S Pd NP catalysts are >95% selective toward the isomer; C8NH2/C6S Pd NPs are 60–75% selective toward the isomer, depending on the ligand ratio; and CnNH2-coated Pd NPs generally produce a 1:1 or 3:2 ratio of the hydrogenation/isomerization products, with a few exceptions. The catalytic turnover frequency (TOF) is low for C6S Pd NPs becaue of the strong thiolate–Pd bond. The TOF increases with increasing chain length in the order C16NH2 Pd > C12NH2 Pd > C8NH2 Pd and increases for Pd91Ag9 alloys compared with pure Pd. The mixed ligand C8NH2/C6S Pd NPs exhibit TOFs similar to pure C8NH2 Pd for low thiol content and similar to C6S Pd NPs for high thiol content. The 130/150 C8NH2/C6S Pd exhibits the optimal TOF for the mixed monolayer Pd NPs. C16NH2 Pd91Ag9 has the highest TOF of all the NPs studied due to the high stability afforded by the bilayer structure of the C16 chain and the high reactivity due to very little interference from the weak metal–amine interaction. Several of the Pd NPs that are stable in the presence of H2 are not stable during the catalysis reaction (H2 plus allyl alcohol), showing that the substrate also plays a role in NP stability.

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The current pandemic has shown that we need sensitive and deployable diagnostic technologies. Surface-enhanced Raman scattering (SERS) sensors can be an ideal solution for developing such advanced point-of-need (PON) diagnostic tests. Homogeneous (reagentless) SERS sensors work by directly responding to the target without any processing step, making them capable for simple one-pot assays, but their limitation is the achievable sensitivity, insufficient compared to what is needed for sensing of viral biomarkers. Noncovalent DNA catalysis mechanisms have been recently exploited for catalytic amplification in SERS assays. These advances used catalytic hairpin assembly (CHA) and other DNA self-assembly processes to develop sensing mechanisms with improved sensitivities. However, these mechanisms have not been used in OFF-to-ON homogeneous sensors, and they often target the same biomarker, likely due to the complexity of the mechanism design. There is still a strong need for a catalytic SERS sensor with a homogeneous mechanism and a rationalization of the catalytic sensing mechanism to translate this sensing strategy to different targets and applications. We developed and investigated a homogeneous SERS sensing mechanism that uses catalytic amplification based on DNA self-assembly. We systematically investigated the role of three domains in the fuel strand (internal loop, stem, and toehold), which drives the catalytic mechanism. The thermodynamic parameters determined in our studies were used to build an algorithm for automated design of catalytic sensors that we validated on target sequences associated with malaria and SARS-CoV-2 strains. With our mechanism, we were able to achieve an amplification level of 20-fold for conventional DNA and of 36-fold using locked nucleic acids (LNAs), with corresponding improvements observed in the sensor limit of detection (LOD). We also show a single-base sequence specificity for a sensor targeting a sequence associated with the omicron variant, tested against a delta variant target. This work on catalytic amplification of homogeneous SERS sensors has the potential to enable the use of this sensing modality in new applications, such as infectious disease surveillance, by improving the LOD while conserving the sensor's homogeneous character.

Red organic dye identification is an important topic for conservation of Japanese ukiyo-e prints. Of particular interest are the works of Suzuki Harunobu, who was working at the inception of full-color printmaking. These prints were made on thin kōzo paper and woodblock printed with semi-transparent to opaque regions of dye(s) and/or pigment(s) mixed with binder. This study used imaging analysis, X-ray fluorescence (XRF), Raman, and surface-enhanced Raman spectroscopies (SERS) to identify the red dyes and pigments on Harunobu prints in the collection of the Portland Art Museum. Through image analysis (visible, UV, and IR illuminations), 23 prints were categorized by appearance. XRF results provided identification of vermilion and ochre pigments, and identified color fields that contained lead. Raman analysis allowed the identification of red lead, and SERS was used to identify both safflower and madder dyes. This work is expected to contribute to the body of knowledge regarding the red dye and pigment palette and mixtures in use in the mid- to late eighteenth century, in the critical early years of full-color printmaking. Graphical Abstract