Near-zero fouling monolayers based on binary patterned peptides allow low nanomolar detection of the matrix metalloproteinase-3 (MMP-3) directly in crude bovine serum, without sample pretreatment, secondary antibody detection or signal amplification. The peptide 3-MPA-HHHDD-OH (3-MPA, 3-mercaptopropionic acid) was found optimal compared to other binary patterned peptides based on 3-MPA-A(x)-B(y)-OH, where 0

Monolayers prepared with polar or ionic amino acids with short side chains have a reduced nonspecific adsorption of serum proteins compared to that of hydrophobic amino acids and organic monolayers immobilized on the gold surface of surface plasmon resonance (SPR) biosensors. Proteins contained in biological samples adsorb on most surfaces, which in the case of biosensors causes a nonspecific response that hinders the quantification of biomarkers in these biological samples. To circumvent this problem, self-assembled monolayers (SAM) of N-3-mercaptopropyl-amino acids (3-MPA-amino acids) were prepared from 19 natural amino acids. These SAM were investigated to limit the nonspecific adsorption of proteins contained in biological fluids and to immobilize molecular receptors (i.e., antibodies) that are necessary in the construction of biosensors. SPR and Ge attenuated total reflection (GATR) FTIR spectroscopy were employed to characterize the formation of the amino acid SAMs. Monolayers of 3-MPA-amino acids densely packed on the surface of the SPR biosensors result in a surface concentration of approximately 10 (15) molecules/cm (2). SPR also quantifies the surface concentration of serum proteins nonspecifically adsorbed on 3-MPA-amino acids following the exposure of the biosensor to undiluted bovine serum. The concentration of nonspecifically bound proteins ranged from approximately 400 ng/cm (2) with polar and ionic amino acids to approximately 800 ng/cm (2) with amino acids of increased hydrophobicity. The nonspecific adsorption of serum proteins on the 3-MPA-amino acids increases in the following order: Asp < Asn < Ser < Met < Glu < Gln < Thr < Gly < His < Cys < Arg < Phe < Trp < Val < Pro < Ile < Leu < Ala < Tyr. The analysis of the adsorption and desorption curves for serum proteins on the SPR sensorgram has demonstrated the strong irreversibility of the protein adsorption on each surface. The effective hydrophilicity of the SAMs was measured from the contact angle with a saline buffer and has demonstrated that surfaces minimizing the contact angle with PBS performed better in serum. The antibody for beta-lactamase was immobilized on a 3-MPA-glycine SAM, and beta-lactamase was detected in the nanomolar range. The presence of beta-lactamase is an indicator of antibiotic resistance.

Short peptides, composed of polar or ionic amino acids, derived with a short organic thiol, significantly reduce nonspecific adsorption of proteins in complex biological matrices such as serum and crude cell lysate, which have nonspecific protein concentrations of 76 and 30-60 mg/mL, respectively. Minimizing these nonspecific interactions has allowed rapid and direct quantification of beta-lactamase in a crude cell lysate using a surface plasmon resonance (SPR) biosensor. A library of short peptides with varying chain length and amino acid composition were synthesized using a solid-phase approach. A 3-mercaptopropionic acid (3-MPA) linker was covalently attached to the amino terminus of the peptides to subsequently form a monolayer on gold in the form of 3-MPA-(AA)(n)-OH, where n is the length of the amino acid chain (n = 2-5). Leu, Phe, Ser, Asp, and His were selected to investigate the effect on nonspecific adsorption with different physicochemical properties of the sidechains; aliphatic, aromatic, polar, acid, and base. Advancing contact angles measured the hydrophobicity of each peptidic self-assembled monolayer (SAM) and showed that hydrophilicity of the gold surface improved as the chain length of the polar or ionic peptides increased, while aromatic and aliphatic peptides decreased the hydrophilicity as the chain length increased. The nonspecific adsorption of undiluted bovine serum on SPR sensors prepared with the library of 3-MPA-(AA)(n)-OH showed that the lowest nonspecific adsorption occurred with polar or ionic amino acids with a chain length of n = 5. We demonstrate that a monolayer composed of 3-MPA-(Ser)(5)-OH has significant advantages, including the following: (1) it minimizes nonspecific adsorption in undiluted bovine serum; (2) it provides a high surface concentration of immobilized antibodies; (3) it shows a great retention of activity for the antibodies; (4) it improves the response from beta-lactamase by approximately 1 order of magnitude, compared to previous experiments; and (5) it allows direct quantification of submicromolar beta-lactamase concentration in a crude cell lysate with a nonspecific protein concentration of 30-60 mg/mL. The use of this peptide-based monolayer offers great advantages for quantitative SPR biosensing in complex biological media.

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A peptide self-assembled monolayer (SAM) was designed to bind His-tagged biomolecules for surface plasmon resonance (SPR) bioanalysis, which was applied for the determination of K(d) for small ligand screening against CD36. Nonspecific adsorption could be minimized using penta- and hexa-peptide monolayers. In particular, monolayers consisting of 3-mercaptopropionyl-leucinyl-histidinyl-aspartyl-leucinyl-histidinyl-aspartic acid (3-Mpa-LHDLHD) exhibited little (12 ng cm(-2)) nonspecific adsorption in crude serum. Modification of this peptide monolayer with Nα,Nα-bis(carboxymethyl)-L-lysine gave a surface competent for binding His-tagged proteins, as demonstrated using enzyme (human dihydrofolate reductase), protein/antibody and receptor (CD36) examples. Immobilization featured chelation of copper and the His-tagged protein by the peptide monolayer, which could be recycled by removing the copper using imidazole washes prior to reuse.

Nanomaterials developed for localized surface plasmon resonance (LSPR) are increasingly integrated to classical prism-based SPR sensors, providing enhanced sensitivity and lower detection limits. The unique properties of these novel nanomaterials in addition to novel surface chemistry to minimize nonspecific adsorption and surface plasmon-coupled techniques with other spectroscopic or mass spectrometry techniques are highlighted in this article.

Metallic thin films patterned with micrometer size triangle or hole arrays present plasmonic properties when excited in the Kretschmann configuration, that are improved in comparison to conventional thin film surface plasmon resonance (SPR). These optical properties can be tuned by varying the physical aspects of the microplasmonic structures. Triangles and microhole arrays were prepared with modified nanosphere lithography (NSL) using latex spheres of 0.65, 0.82, 1.0, 1.5, or 3.2 microm in diameter. This allowed the preparation of triangles with edge lengths between 275 to 2000 nm and microhole arrays of various periodicities, diameters, and hole depths. These microstructures were studied to understand the relationship between the physical aspects and the optical properties, such as the sensitivity, working refractive index range, spectral width of the plasmonic peaks, spectral noise, and refractive index resolution. Microhole arrays with a hole diameter equal to half the periodicity were found to combine the advantages of both localized surface plasmon resonance (LSPR) on nanoparticles and SPR on a thin film. These microhole arrays exhibited high sensitivity to refractive index (>3000 nm/RIU), sensitivity to monolayer formation (2-fold improvement compared to thin films), and excellent refractive index resolution (10(-6) RIU). Finally, a biosensor for the detection of 10 nM of immunoglobulin G (IgG) exhibited a greater response with microplasmonic materials compared to conventional thin Au films. Hence, these novel plasmonic materials exhibit a strong potential as an SPR sensing platform. They can be implemented on existing instrumentation and use detection protocols developed for current SPR sensors.

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