Immobilized single-stranded DNA (ssDNA) can be used as a selective "reagent" to bind complementary nucleic acids for applications including detection of pathogenic organisms and genetic mutations. The density of ssDNA on a surface will determine nearest neighbor interactions, surface interactions, and charge density due to ionizable phosphate groups. This may result in a local ionic strength, pH, and dielectric constant at the surface that is substantially different from that in bulk electrolyte solution. It is the local conditions that influence the thermodynamics of hybridization, and this can be studied by the melt temperature (Tm) of double-stranded DNA (dsDNA). Organosilane chemistry has been used to covalently immobilize hexaethylene glycol linkers and to control the subsequent density of dT20 that was prepared by automated synthesis. Fiber-optic biosensors based on fused silica optical fibers that were coated with DNA were used in a total internal reflection fluorescence instrument to determine Tm from the dissociation of duplexes of mixtures of fluorescein-labeled and unlabeled dA20 and d(A9GA10). Each thermal denaturation of dsDNA at the surface of the optical fibers was accompanied by a 2-3-fold reduction in standard enthalpy change, relative to values determined for denaturation in bulk solution. The experimental results suggest that the thermodynamic stability of duplexes that are immobilized on a surface is dependent on the density of immobilized DNA. Additionally, the deviation in Tm arising as a result of the presence of a centrally located single base-pair mismatch was significantly larger for thermal denaturation occurring at the surface of the optical fibers (∆Tm ) 6-10 °C) relative to that observed in bulk solution (∆Tm ) 3.8-6.1 °C). These results suggest that hybridization at an interface occurs in a significantly different physical environment in comparison to hybridization in bulk solution, and that surface density can be tuned to design analytical figures of merit.

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Enzyme-linked immunosorbent assay (ELISA) and liquid chromatography tandem mass spectrometry (LC/MS/MS) were used to detect diazepam exposure in skeletal tissues of rats (n = 15) given diazepam acutely (20 mg/kg, i.p.), and killed at various times postdose. Marrow, epiphyseal, and diaphyseal bone were isolated from extracted femora. Bone was cleaned, ground, and incubated in methanol. Marrow underwent ultrasonic homogenization. Extracts and homogenates were diluted in phosphate buffer, and then underwent solid-phase extraction and ELISA. Relative sensitivity of detection was examined in terms of relative decrease in absorbance (ELISA) and binary classification sensitivity (ELISA and LC/MS/MS). Overall, the data showed differences in relative sensitivity of detection of diazepam exposure in different tissue types (marrow > epiphyseal bone > diaphyseal bone), which is suggestive of heterogenous distribution in these tissues, and a decreasing sensitivity with increasing dose-death interval. Thus, the tissue type sampled and dose-death interval may contribute to the probability of detection of diazepam exposure in skeletal tissues.

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Ketamine exposure was detected in skeletal tissues by ELISA and liquid chromatography-tandem mass spectrometry (LC-MS-MS). Rats (n = 9) received ketamine hydrochloride acutely (75 mg/kg, i.p.) and were euthanized within 15, 30, or 90 min. Drug-free control animals (n = 3) were also euthanized. Extracted femora were separated into epiphyseal and diaphyseal fragments, with marrow isolated from the medullary cavity. Bone was ground and incubated in methanol. Extracts were dried and reconstituted in phosphate buffer (0.1 M, pH 7.3), and marrow was homogenized in alkaline solution. Both then underwent solid-phase extraction. Extracts were assayed by ELISA, with data expressed in terms of relative decrease in absorbance (%DA, drug-positive tissues vs. matrix-matched drug-free controls) and binary classification test sensitivity (S). Generally, %DA decreased in the order of marrow > epiphyseal bone > diaphyseal bone, and was negatively correlated with dose-death interval (DDI). Measured S values were 100% in ELISA analysis of extracts of all tissue types. Sensitivity values were computed from LC-MS-MS data using a 5 ng/mL cutoff. Sensitivity values for ketamine detection were 100%, 0-100% and 0%, at the 15, 30, and 90 min DDI, respectively, and sensitivity values for norketamine detection were 0-66%, 0-66%, and 0% at the 15, 30, and 90 min DDI, respectively. These results suggest that the tissue type sampled and DDI may influence the sensitivity of detection of ketamine exposure in skeletal tissues.

Detection of ketamine exposure in skeletal tissues by automated enzyme-linked immunosorbent assay (ELISA) and gas chromatography with electron capture detection (GC-ECD) is described. Rats (n = 18) received 0, 15, 30, or 75 mg/kg ketamine hydrochloride acutely (i.p.), and were euthanized within 15 min or 1 h. Ketamine was extracted from ground femoral bone by methanolic incubation followed by liquid-liquid extraction (LLE), while marrow was homogenized in alkaline solution, and then underwent LLE. Extracts were analyzed by ELISA, and subsequently by GC-ECD following derivatization with trifluoroacetic acid anhydride. The effect of tissue type (i.e., diaphyseal bone vs. epiphyseal bone vs. bone marrow) on the immunoassay response was examined through determination of binary classification test sensitivity (S) and measurement of the relative decrease in absorbance (%DA, drug-positive tissues vs. drug-free controls) in each tissue type. The %DA varied significantly between different tissues examined under a given dose condition, and generally decreased in the order marrow > epiphyseal bone > diaphyseal bone, at all dose levels examined. Measured S values for marrow, epiphyseal bone, and diaphyseal bone were 100%, 77%, and 23%, respectively (75 mg/kg dose). These results suggest that the type of skeletal tissue sampled and position sampled within a given bone (diaphyses vs. epiphyses) are important parameters in drug screening of skeletal tissues.