Generalized Ratiometric Indicator Based Surface-Enhanced Raman Spectroscopy for the Detection of Cd²⁺ in Environmental Water Samples

The concept of generalized ratiometric indicator based surface-enhanced Raman spectroscopy was first introduced and successfully implemented in the detection of Cd²⁺ in environmental water samples using Au nanoparticles (AuNPs) modified by trithiocyanuric acid (TMT). Without the use of any internal standard, the proposed method achieved accurate concentration predictions for Cd²⁺ in environmental water samples with recoveries in the ranges of 91.8–108.1%, comparable to the corresponding values obtained by atomic absorption spectroscopy. The limit of detection and limit of quantification were estimated to be 2.9 and 8.7 nM, respectively. More importantly, other species present in water samples which cannot react with TMT and have weaker binding ability to AuNPs than TMT do not interfere with the quantification of Cd²⁺. Therefore, it is expected that the combination of the generalized ratiometric indicator based surface-enhanced Raman spectroscopy with the proposed AuNP–TMT probing system can be a competitive alternative for the primary screening of Cd²⁺ pollution

The large position displacement correlation coefficients (<i>r_i,j</i>>0.60) in the PDZ domain database.

<p>The large position displacement correlation coefficients (<i>r_i,j</i>>0.60) in the PDZ domain database.</p

Structure of PDZ domain 1BE9 and multiple structural alignment (MSA) of 186 PDZ domains.

<p>(<b>A</b>) The structure of PDZ domain 1BE9 and peptide ligand. Target C-terminal ligands bind in a surface groove formed between the α2 helix and the β2 strand at a number of binding sites that determine both ligand affinity and sequence specific recognition. Blue is for hydrophilic surface and green for hydrophobic surface. (<b>B</b>) The multiple structural alignment (MSA) database of 186 PDZ crystal structures. PDZ domains consist of 90–100 residues that adopt a six-stranded β sandwich configuration with two flanking α helices. In the MSA database there are 117 residue positions, including gaps inserted in structural alignment. After deletion of unnecessary gaps, the length of MSA database is 96 positions. (<b>C</b>) The locations of 6 structural segments and the secondary structural units of PDZ protein domains. The four PDZ protein domains (2QKT, 2F5Y, 1G9O, and 1BE9) are taken from the MSA database of 186 PDZ domains. The six structural segments (S1 to S6) are indicated by green frameworks, and the secondary structural units (α-helices, β-strands, and loops) are indicated by color bars (blue for loops, yellow for β-strands, and red for α-helices). The structural segments are stable units in the structural changes of protein family.</p

The flowchart of structural position correlation analysis (SPCA).

<p>The displacement matrix D^(α)_N×L and D^(m)_N×L is the distant differences between standard protein P⁽⁰⁾ and proteins of protein evolutionary family. The superscripts ‘α’ and ‘m’ indicate the α-carbon and mass center, respectively. From the statistical correlation analysis to the residue position displacements D^(α)_N×L, the residue positions are reorganized as structural segments. Then statistical correlation analysis is applied to the structural segment displacement matrix D^(s)_N×K, revealing the segment correlation information of functional evolution in the protein family.</p

The most conservative positions^* in PDZ domain database.

<p>*The frequency of residue <i>k</i> at the most conservative position <i>l</i> is larger than 0.80, f^(m)<i>_k,l</i>>0.80.</p

The displacement correlation relationships between structural segments and positions.

<p>(<b>A</b>) The displacement correlation between segments S2 (in β2) and S5 (in α2). The correlation of S2 and S5, actually, represents the structural correlation between α2 and β2. (<b>B</b>) The displacement correlation between position 37 (in β3) and position 78 (α2). The correlation of positions 37 and 78 causes a distant allosteric interaction in the PDZ domain.</p

Physical conditions of HTC reactions and yields of organic products and solid carbon.

<p>Physical conditions of HTC reactions and yields of organic products and solid carbon.</p

GC-MS spectra of organic byproducts from HTC reactions of cellulose at three temperatures.

<p>(a) 200, (b) 250,(c) 300°C.</p

The chemical structures of cellulose and lignin.

<p>(a) Cellulose is a carbohydrate polymer consisting of glucose monomer through 1–4 glycoside bonds. (b) Lignin is cross linked phenolic polymer, consisting of three phenol monomers, coumaryl alcohol, coniferyl alcohol, and syringyl alcohol.</p

Illustration of bio-refinery of sugarcane bagasse.

<p>In above scheme the hemicellulose is used to prepare xylan, a very valuable medicine for hypertensive and diabetic patients. The cellulose is a widely-used material for pulp, glucose, and ethanol. The lignin is rich in phenolic compounds, the raw materials for many chemical products.</p