A global benchmark study using affinity-based biosensors

To explore the variability in biosensor studies, 150 participants from 20 countries were given the same protein samples and asked to determine kinetic rate constants for the interaction. We chose a protein system that was amenable to analysis using different biosensor platforms as well as by users of different expertise levels. The two proteins (a 50-kDa Fab and a 60-kDa glutathione S-transferase [GST] antigen) form a relatively high-affinity complex, so participants needed to optimize several experimental parameters, including ligand immobilization and regeneration conditions as well as analyte concentrations and injection/dissociation times. Although most participants collected binding responses that could be fit to yield kinetic parameters, the quality of a few data sets could have been improved by optimizing the assay design. Once these outliers were removed, the average reported affinity across the remaining panel of participants was 620 pM with a standard deviation of 980 pM. These results demonstrate that when this biosensor assay was designed and executed appropriately, the reported rate constants were consistent, and independent of which protein was immobilized and which biosensor was used.

Looking towards label-free biomolecular interaction analysis in a high-throughput format: a review of new surface plasmon resonance technologies

Surface plasmon resonance (SPR) biosensors have enabled a wide range of applications in which researchers can monitor biomolecular interactions in real time. Owing to the fact that SPR can provide affinity and kinetic data, unique features in applications ranging from protein-peptide interaction analysis to cellular ligation experiments have been demonstrated. Although SPR has historically been limited by its throughput, new methods are emerging that allow for the simultaneous analysis of many thousands of interactions. When coupled with new protein array technologies, high-throughput SPR methods give users new and improved methods to analyze pathways, screen drug candidates and monitor protein-protein interactions.

Controlling DNA orientation on mixed ssDNA/OEG SAMs

We present and characterize a mixed self-assembled monolayer (SAM) consisting of single-stranded oligonucleotide (ssDNA)- and oligo(ethylene glycol) (OEG)-terminated thiols. The ssDNA/OEG SAMs are prepared by simultaneous coadsorption from a common thiol solution over a broad range of compositions. Electron spectroscopy for chemical analysis (ESCA) is used to measure the surface coverage of ssDNA, whereas surface plasmon resonance (SPR) sensor is used to measure the hybridization of complementary ssDNA and protein resistance. Through the complementary use of these techniques, we find that the composition of OEG in the assembly solution controls a key parameter: the surface coverage of ssDNA on the surface. There is evidence that it influences the orientation of the immobilized ssDNA probes. Lower OEG concentrations yield a surface with higher ssDNA coverage and less favorable orientation, whereas higher OEG concentrations produce a surface with lower DNA coverage and more favorable orientation. Competition between these two effects controls the hybridization efficiency of the ssDNA surface. Compared to ssDNA surfaces prepared with other diluent thiols, the use of OEG improves the protein resistance of the surface, making it more broadly applicable.

DNA-Directed Protein Immobilization for Simultaneous Detection of Multiple Analytes by Surface Plasmon Resonance Biosensor

A versatile multichannel biosensor surface is prepared by site-directed immobilization of single-stranded DNA-protein conjugates onto a patterned self-assembled monolayer composed of ssDNA thiols and oligo(ethylene glycol)-terminated thiols. The conjugates each consist of an antibody chemically linked to a unique ssDNA target with a sequence complementary to the surface-bound ssDNA probes and are immobilized on the surface via sequence-specific hybridization. The exceptional specificity of DNA hybridization combined with the diversity of available sequences makes this platform perfect for multichannel sensors. Once the surface is patterned with the appropriate probe sequences, sequence-specific hybridization sorts out the target conjugates and directs them to the appropriate spots on the surface. Previously (Boozer, C. L.; Ladd, J.; Chen, S.; Yu, Q.; Homola, J.; Jiang, S. Anal. Chem. 2004, 76, 6967-6972), we performed proof-of-concept experiments demonstrating the feasibililty of using DNA-directed protein immobilization to produce a single channel biosensor. In this work, we extend this technique and employ DNA-directed protein immobilization to functionalize a multichannel biosensor, which was used for the simultaneous detection of a set of three fertility hormones: human chorionic gonadotropin, human luteinizing hormone, and follicle stimulating hormone by surface plasmon resonance sensor.

DNA Directed Protein Immobilization on Mixed ssDNA/Oligo(ethylene glycol) Self-Assembled Monolayers for Sensitive Biosensors

A stable and versatile biosensor surface is prepared by site-directed immobilization of protein-DNA conjugates onto a mixed self-assembled monolayer (SAM) composed of ssDNA thiols and oligo(ethylene glycol) (OEG) terminated thiols. The protein conjugates consist of an antibody chemically linked to a ssDNA target with a sequence complementary to the surface-bound ssDNA probes and are immobilized on the surface via sequence-specific hybridization. Compared to standard antibody immobilization techniques, this approach offers many advantages. The exceptional specificity of DNA hybridization combined with the diversity of potential sequences makes this platform perfect for multichannel sensors. Once a surface is patterned with the appropriate probe sequences, sequence-specific hybridization will sort out the target conjugates and direct them to the appropriate spots on the surface. In addition, the DNA SAMs are very stable and well suited to recycling by dehybdridization of the conjugates from the surface-bound probes. In this work, we demonstrate the specificity, sensitivity, and convenience of using protein-DNA conjugates to convert a DNA/OEG SAM surface into a biosensor surface and apply this platform to the detection of human chorionic gonadotropin using surface plasmon resonance.

DNA-directed protein immobilization on mixed self-assembled monolayers via a streptavidin bridge

The simultaneous detection of multiple analytes is an important consideration for the advancement of biosensor technology. Currently, few sensor systems possess the capability to accurately and precisely detect multiple antigens. This work presents a simple approach for the functionalization of sensor surfaces suitable for multichannel detection. This approach utilizes self-assembled monolayer (SAM) chemistry to create a nonfouling, functional sensor platform based on biotinylated single-stranded DNA immobilized via a streptavidin bridge to a mixed SAM of biotinylated alkanethiol and oligo(ethylene glycol). Nonspecific binding is minimized with the nonfouling background of the sensor surface. A usable protein chip is generated by applying protein-DNA conjugates which are directed to specific sites on the sensor chip surface by utilizing the specificity of DNA hybridization. The described platform is demonstrated in a custom-built surface plasmon resonance biosensor. The detection capabilities of a sensor using this protein array have been characterized using human chorionic gonadotropin (hCG). The platform shows a higher sensitivity in detection of hCG than that observed using biotinylated antibodies. Results also show excellent specificity in protein immobilization to the proper locations in the array. The vast number of possible DNA sequences combine with the selectivity of base-pairing makes this platform an excellent candidate for a sensor capable of multichannel protein detection.

Physiological response to long-term peripheral and central leptin infusion in lean and obese mice

Recent data have identified leptin as an afferent signal in a negative-feedback loop regulating the mass of the adipose tissue. High leptin levels are observed in obese humans and rodents, suggesting that, in some cases, obesity is the result of leptin insensitivity. This hypothesis was tested by comparing the response to peripherally and centrally administered leptin among lean and three obese strains of mice: diet-induced obese AKR/J, New Zealand Obese (NZO), and Ay. Subcutaneous leptin infusion to lean mice resulted in a dose-dependent loss of body weight at physiologic plasma levels. Chronic infusions of leptin intracerebroventricularly (i.c.v.) at doses of 3 ng/hr or greater resulted in complete depletion of visible adipose tissue, which was maintained throughout 30 days of continuous i.c.v. infusion. Direct measurement of energy balance indicated that leptin treatment did not increase total energy expenditure but prevented the decrease that follows reduced food intake. Diet-induced obese mice lost weight in response to peripheral leptin but were less sensitive than lean mice. NZO mice were unresponsive to peripheral leptin but were responsive to i.c.v. leptin. Ay mice did not respond to subcutaneous leptin and were 1/100 as sensitive to i.c.v. leptin. The decreased response to leptin in diet-induced obese, NZO, and Ay mice suggests that obesity in these strains is the result of leptin resistance. In NZO mice, leptin resistance may be the result of decreased transport of leptin into the cerebrospinal fluid, whereas in Ay mice, leptin resistance probably results from defects downstream of the leptin receptor in the hypothalamus.

Presence of anti-hepatitis C virus serum markers in a dental school patient population

Hepatitis C virus (HCV) has joined the list of infections liver diseases of which the practicing dentist must be aware. This recently described RNA virus causes between 20% to 40% of reported cases of viral hepatitis in the United States. Patients with HCV show symptoms only one quarter of the time, but chronicity is maintained longer than with the other liver infections. The major mode of transmission is percutaneous, so dentists are potentially at risk to contact the virus. To assess the presence of exposure to HCV, a total of 500 dental school patients were screened for serum antibody to this virus (second-generation anti-HCV). In addition, participants were required to fill out a special screening questionnaire designed to indicate high-risk groups. Their serum was also analyzed for serum chemistries known to be associated with acute and chronic liver disease. Analysis of the 15 response questionnaires revealed statistical significance relating to questions inquiring about previous exposure to blood (transfusions, surgical procedures, etc) but little significance relating to lifestyle. In either case responses were not of practical predictive value. More than 5% of our sample patients were found to be positive reactors to anti-HCV, indicating previous HCV exposure. A significant number of those positive for anti-HCV also showed elevation of serum chemistries associated with hepatitis.