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Biconically tapered fiber optic probes for rapid label-free immunoassays.

Miller J, Castaneda A, Lee KH, Sanchez M, Ortiz A, Almaz E, Almaz ZT, Murinda S, Lin WJ, Salik E - Biosensors (Basel) (2015)

Bottom Line: Hydrofluoric acid treatment makes the sensitive region thinner to enhance sensitivity, which we confirmed by experiments and simulations.The limit of detection for the sensor was estimated to be less than 50 ng/mL.Utilization of the rate of the sensor peak shift within the first few minutes of the antibody-antigen reaction is proposed as a rapid protein detection method.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, University of California, Los Angeles, 475 Portola Plaza, Los Angeles, CA 90095, USA. johnmiller@physics.ucla.edu.

ABSTRACT
We report use of U-shaped biconically tapered optical fibers (BTOF) as probes for label-free immunoassays. The tapered regions of the sensors were functionalized by immobilization of immunoglobulin-G (Ig-G) and tested for detection of anti-IgG at concentrations of 50 ng/mL to 50 µg/mL. Antibody-antigen reaction creates a biological nanolayer modifying the waveguide structure leading to a change in the sensor signal, which allows real-time monitoring. The kinetics of the antibody (mouse Ig-G)-antigen (rabbit anti-mouse IgG) reactions was studied. Hydrofluoric acid treatment makes the sensitive region thinner to enhance sensitivity, which we confirmed by experiments and simulations. The limit of detection for the sensor was estimated to be less than 50 ng/mL. Utilization of the rate of the sensor peak shift within the first few minutes of the antibody-antigen reaction is proposed as a rapid protein detection method.

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Sensor response for three different concentrations of anti-IgG: (a) 0.5 µg/mL; (b) 5.0 µg/mL; (c) 50 µg/mL; Note that different wavelength scales are used to show the details of the dependence. The time scale is expressed using scale bars. The data fit best to double exponential functions; (d) Comparison of the peak shift in the first few minutes of the anti-IgG binding showing different rates for different concentrations.
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biosensors-05-00158-f005: Sensor response for three different concentrations of anti-IgG: (a) 0.5 µg/mL; (b) 5.0 µg/mL; (c) 50 µg/mL; Note that different wavelength scales are used to show the details of the dependence. The time scale is expressed using scale bars. The data fit best to double exponential functions; (d) Comparison of the peak shift in the first few minutes of the anti-IgG binding showing different rates for different concentrations.

Mentions: Figure 5 shows the comparison of the spectral shifts with time for three concentrations (0.5, 5, and 50 µg/mL) of the anti-IgG solution. Each of these sensors was monitored for at least 30 min during IgG-anti-IgG binding. All of them show permanent peak shifts as a result of IgG-anti-IgG binding. For the 50 µg/mL anti-IgG test, we observed a very sharp shift right after the sensor was immersed in the anti-IgG solution. Given that we can collect a data set once in every 20 s, we could not capture the initial data points. We expect this jump is due to very rapid binding at the high IgG concentration. But the sudden change in the average refractive index is also worth analyzing. It is well established that the refractive index of protein solutions is proportional to concentration:(6)nps=ns+a×Cwhere nps and ns are the refractive indices of the protein solution and the base solution, respectively [21]. If C is the concentration of the protein in grams/(100 mL), then the proportionality constant a becomes about 2 × 10−3 mL/g. Using this relationship, we compute the change in refractive index (nps − ns) to be ~10−5, which is expected to cause a peak shift of <0.1 nm. However, we measured a ~1.3 nm sudden shift. This is very hard to explain by mere average refractive index change around the sensor. Therefore, a permanent protein nanolayer should be forming on the sensor surface extremely quickly. The measurements in PBST solutions before and after the IgG-anti-IgG binding stage also confirm this conclusion. Therefore, the sensing mechanism is a surface mechanism, as expected of biosensors.


Biconically tapered fiber optic probes for rapid label-free immunoassays.

Miller J, Castaneda A, Lee KH, Sanchez M, Ortiz A, Almaz E, Almaz ZT, Murinda S, Lin WJ, Salik E - Biosensors (Basel) (2015)

Sensor response for three different concentrations of anti-IgG: (a) 0.5 µg/mL; (b) 5.0 µg/mL; (c) 50 µg/mL; Note that different wavelength scales are used to show the details of the dependence. The time scale is expressed using scale bars. The data fit best to double exponential functions; (d) Comparison of the peak shift in the first few minutes of the anti-IgG binding showing different rates for different concentrations.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4493543&req=5

biosensors-05-00158-f005: Sensor response for three different concentrations of anti-IgG: (a) 0.5 µg/mL; (b) 5.0 µg/mL; (c) 50 µg/mL; Note that different wavelength scales are used to show the details of the dependence. The time scale is expressed using scale bars. The data fit best to double exponential functions; (d) Comparison of the peak shift in the first few minutes of the anti-IgG binding showing different rates for different concentrations.
Mentions: Figure 5 shows the comparison of the spectral shifts with time for three concentrations (0.5, 5, and 50 µg/mL) of the anti-IgG solution. Each of these sensors was monitored for at least 30 min during IgG-anti-IgG binding. All of them show permanent peak shifts as a result of IgG-anti-IgG binding. For the 50 µg/mL anti-IgG test, we observed a very sharp shift right after the sensor was immersed in the anti-IgG solution. Given that we can collect a data set once in every 20 s, we could not capture the initial data points. We expect this jump is due to very rapid binding at the high IgG concentration. But the sudden change in the average refractive index is also worth analyzing. It is well established that the refractive index of protein solutions is proportional to concentration:(6)nps=ns+a×Cwhere nps and ns are the refractive indices of the protein solution and the base solution, respectively [21]. If C is the concentration of the protein in grams/(100 mL), then the proportionality constant a becomes about 2 × 10−3 mL/g. Using this relationship, we compute the change in refractive index (nps − ns) to be ~10−5, which is expected to cause a peak shift of <0.1 nm. However, we measured a ~1.3 nm sudden shift. This is very hard to explain by mere average refractive index change around the sensor. Therefore, a permanent protein nanolayer should be forming on the sensor surface extremely quickly. The measurements in PBST solutions before and after the IgG-anti-IgG binding stage also confirm this conclusion. Therefore, the sensing mechanism is a surface mechanism, as expected of biosensors.

Bottom Line: Hydrofluoric acid treatment makes the sensitive region thinner to enhance sensitivity, which we confirmed by experiments and simulations.The limit of detection for the sensor was estimated to be less than 50 ng/mL.Utilization of the rate of the sensor peak shift within the first few minutes of the antibody-antigen reaction is proposed as a rapid protein detection method.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, University of California, Los Angeles, 475 Portola Plaza, Los Angeles, CA 90095, USA. johnmiller@physics.ucla.edu.

ABSTRACT
We report use of U-shaped biconically tapered optical fibers (BTOF) as probes for label-free immunoassays. The tapered regions of the sensors were functionalized by immobilization of immunoglobulin-G (Ig-G) and tested for detection of anti-IgG at concentrations of 50 ng/mL to 50 µg/mL. Antibody-antigen reaction creates a biological nanolayer modifying the waveguide structure leading to a change in the sensor signal, which allows real-time monitoring. The kinetics of the antibody (mouse Ig-G)-antigen (rabbit anti-mouse IgG) reactions was studied. Hydrofluoric acid treatment makes the sensitive region thinner to enhance sensitivity, which we confirmed by experiments and simulations. The limit of detection for the sensor was estimated to be less than 50 ng/mL. Utilization of the rate of the sensor peak shift within the first few minutes of the antibody-antigen reaction is proposed as a rapid protein detection method.

Show MeSH
Related in: MedlinePlus