Strong Resistance of Citrate Anions on Metal Nanoparticles to Desorption under Thiol Functionalization

Thiols are widely utilized to functionalize metal nanoparticles, including ubiquitous citrate-stabilized gold nanoparticles (AuNPs), for fundamental studies and biomedical applications. For more than two decades, citrate-to-thiol ligand exchange has been used to introduce functionality to AuNPs in the 5–100 nm size regime. Contrary to conventional assumptions about the completion of ligand exchange processes and formation of a uniform self-assembled monolayer (SAM) on the NP surface, coadsorption of thiols with preadsorbed citrates as a mixed layer on AuNPs is demonstrated. Hydrogen bonding between carboxyl moieties primarily is attributed to the strong adsorption of citrate, leading to the formation of a stabilized network that is challenging to displace. In these studies, adsorbed citrates, probed by Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) analyses, remain on the surface following thiol addition to the AuNPs, whereas acetoacetate anions are desorbed. XPS quantitative analysis indicates that the surface density of alkyl and aryl thiolates for AuNPs with an average diameter of ∼40 nm is 50–65% of the value of a close-packed SAM on Au(111). We present a detailed citrate/thiolate coadsorption model that describes this final mixed surface composition. Intermolecular interactions between weakly coordinated oxyanions, such as polyprotic carboxylic acids, can lead to enhanced stability of the metal–ligand interactions, and this needs to be considered in the surface modification of metal nanoparticles by thiols or other anchor groups

Structural Study of Citrate Layers on Gold Nanoparticles: Role of Intermolecular Interactions in Stabilizing Nanoparticles

The structure of citrate adlayers on gold nanoparticles (AuNPs) was investigated. Infrared (IR) and X-ray photoelectron spectroscopy (XPS) analyses indicate citrate anions are adsorbed on AuNPs through central carboxylate groups. A unique structure of adsorbed citrate is determined, and a pH-induced structural transition is presented. IR analysis probes dangling dihydrogen anions (H₂Citrate^–) and hydrogen bonding of carboxylic acid groups between adsorbed and dangling citrate anions. A contribution of steric repulsion between citrate layers to particle stability is characterized. Structure-based modeling, which is consistent with scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) images in the literature, suggests organization details relating to the formation of self-assembled layers on (111), (110), and (100) surfaces of AuNPs. Adsorption characteristics of the citrate layer include the interaction between hydrogen-bonded citrate chains, bilayer formation, surface coverage, and chirality. The enthalpic gain from intermolecular interactions and the importance of molecular structure/symmetry on the adsorption are discussed. Combining the enthalpic factor with surface diffusion and adsorption geometry of (1,2)-dicarboxyl fragments on Au(111), H₂Citrate^– anions effectively stabilize the (111) surface of the AuNPs. The detailed understanding of intermolecular interactions in the molecular adlayer provides insight for nanoparticle formation and stabilization. We expect these findings will be relevant for other nanoparticles stabilized by hydroxy carboxylate-based amino acids and have broad implications in NP-based interfacial studies and applications

MOESM1 of A novel Sugarcane bacilliform virus promoter confers gene expression preferentially in the vascular bundle and storage parenchyma of the sugarcane culm

Additional file 1: Table S1. List of primers used for cloning the SCBV21 promoter and its deletions

Expression profile of <i>SspNIP2</i> in <i>S</i>. <i>spontaneum</i> TUS05-05 during 24 hrs of cold treatment.

<p>Time course experiment was conducted by qRT-PCR to study the dynamics of SspNIP2 gene expression profile during cold treatment. Five TUS05-05 plants were subjected to cold treatment at 0°C, and leaf tissue samples were collected at different time point as shown on the Fig for RNA preparation. RNA samples from five individual plants were pooled for qRT-PCR. qRT-PCR was conducted three times with triplicates each time. RT indicated the RNA sample prepared from plants that were returned to normal growth condition after cold treatment.</p

MOESM1 of A novel Sugarcane bacilliform virus promoter confers gene expression preferentially in the vascular bundle and storage parenchyma of the sugarcane culm

Additional file 1: Table S1. List of primers used for cloning the SCBV21 promoter and its deletions

Analysis of gene annotation of differentially expressed genes after chilling stress.

<p>The annotated sequences of CP72-1210 (top) and TUS05-05 (bottom) were analyzed based on the molecular function of GO terms by Blast2Go. GO terms are listed on the left, and the Blast2Go score of molecular function at level 3 is shown on top. The molecular function of transmembrane transporter activities are underlined in red.</p

Water stress test on SspNIP2 transgenic lines.

<p>Relative moisture content (%) retained in each potted plant was measured during the first six days of water stress. The designation of transgenic lines are the same as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125810#pone.0125810.g005" target="_blank">Fig 5</a>. NIP2 #6-#7 and #8-#9 indicated that SspNIP2 transgenic lines #6 and #7, and lines #8 and #9 were combined, respectively, for the analysis. Statistical analysis was conducted by F-test followed by t-test.</p

Salt stress test on SspNIP2 transgenic lines.

<p>Transgenic plants were irrigated with 100 ml of 200 mM NaCl for 4 weeks and 250 mM NaCl for another 2 weeks. Arrows indicate non-transgenic lines. NIP2 #6, #7, #8 and #9 referred to SspNIP2 transgenic line 6, 7, 8 and 9.</p

Summary of RNA-Seq data of sugarcane CP72-1210 and <i>S</i>. <i>spontaneum</i> TUS05-05.

<p>^aM = Million reads</p><p>^bSAS = Sugarcane Assembled Sequences from SUCEST-FUN Database (<a href="http://sucest-fun.org" target="_blank">http://sucest-fun.org</a>) at Instituto de Química—Universidade de São Paulo, funded by FAPESP Bioenergy Research Program BIOEN (<a href="http://bioenfapesp.org" target="_blank">http://bioenfapesp.org</a>).</p><p>Summary of RNA-Seq data of sugarcane CP72-1210 and <i>S</i>. <i>spontaneum</i> TUS05-05.</p

Venn diagram of differentially expressed genes in CP72-1210 and TUS05-05.

<p><b>A.</b> Venn diagram of differentially expressed SAS sequences before gene annotation. <b>B.</b> Venn diagram of annotated genes differentially expressed in each genotype. The number of uniquely and commonly expressed genes in each genotype is indicated in numbers in the diagram. Up and Down indicate up-regulation and down-regulation, respectively.</p