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METAL ENHANCED FLUORESCENCE (MEF)

Enhanced Fluorescent Signal:

Figure 1.  Same amount of fluorophore on untreated surface (top) and on plasmonic surface (bottom)

Figure 1. Same amount of fluorophore on untreated surface (top) and on plasmonic surface (bottom)

When a fluorophore is brought in close proximity to a metal nanoparticle, the fluorescence intensity of the fluorophore has been shown to increase dramatically (see Figure 1) and the fluorescence lifetime of the fluorophore has been shown to decrease, resulting in an increased photostability (1,2). This phenomenon is known as Metal Enhanced Fluorescence (MEF), a term first used by Geddes and Lakowicz in 2002 (1).  The resulting increase in signal intensity is the basis for Plasmonix’ products which allow our customers to detect levels of material previously undetectable by fluorescence.

Since its discovery, the physical basis for MEF has been well studied and reviewed (2, 3, 4, 5). It is now  generally accepted that MEF is caused by the non-radiative coupling of the fluorophore dipole with the electron cloud of the metal (surface plasmons),thereby altering the fluorescence characteristics. This is schematized in Figure 2.

MEF is a near-field phenomenon, meaning that it occurs only when a fluorophore is within a fraction of the incoming propagating wavelength (i.e. 5 – 50 nm) of a metal which supports surface plasmons (5).  Beyond this distance, fluorophores are un-affected by metal and display their characteristic and classical well known far-field fluorescence. MEF can be induced by a wide variety of metals including silver, gold, tin, copper and nickel(2,6).  Each metal however displays distinct spectral characteristics as it enhances fluorescence (2,6).  Very recently gold-coated slides were used to develop a diagnostic assay for Type 1 diabetes (14)

Plasmonix’ Applications.  Plasmonix is creating a series of new products that take advantage of Metal Enhanced Fluorescence.  Our QuantaWell 2 plates enhance immunoassay fluorescence by 20 – 50-fold and, by using a dye-conjugated primary antibody, allow immunoassays to be completed in 1-2 hours instead of the usual 4 – 6 hours needed to run an ELISA.  Our QuantArray slides for proteomic applications enhance  fluorescence signals by 100 – 200-fold and allow detection of proteins and antibodies not otherwise detectable.   Plasmonix continues to develop innovative products with enhanced fluorescent detection and expects to release fluorescent metal nanoparticles for solution-based immunoassay applications in the near future.

Other Applications.  MEF has found numerous applications in enhancing the sensitivity of a wide variety of fluorescence assays (2) including immunoassays (7), chemiluminescence assays (8), nucleic acid assays (9), proteomics (10) and protein release assays in cells (11).  It has also been used to devise ultra-rapid and sensitive detection of Salmonella (12), and is currently in clinical trials as an incredibly inexpensive and rapid test for Chlamydia (13) and Gonorrhea.  We invite interested readers to consult the growing scientific literature for many additional applications of MEF, or to contact us for further scientific information.

Figure 2.  Schematic representation of MEF. A fluorophore induces a mirror dipole in the metals’ surface plasmons.  The metal in-turn radiates the coupled quanta.

Figure 2. Schematic representation of MEF. A fluorophore induces a mirror dipole in the metals’ surface plasmons. The metal in-turn radiates the coupled quanta.

REFERENCES

  1. C. D. Geddes and J.R. Lakowicz, Metal-enhanced fluorescence, J. Fluoresc., (2002), 12: 121-129.
  2. Metal-Enhanced Fluorescence, ed. C.D. Geddes, John Wiley and Sons, New Jersey, 2010, pp. 625, ISBN: 9780-470-2238-8
  3. Aslan, K., Gryczynski, I., Malicka, J., Matveeva, E., Lakowicz, J.R. and Geddes, C.D. (2005). Metal-enhanced fluorescence: an emerging tool in biotechnology, Current Opinion in Biotechnology, 16: 55-62.
  4. Plasmon-controlled fluorescence towards high-sensitivity optical sensing. Ray K, Howdhury MH, Zhang J, Fu Y, Szmacinski H, Nowaczyk, K, Lakowicz JR. Adv Biochem Eng Biotechnol (2009) 116: 29-72.
  5. Advances in surface-enhanced fluorescence. Lakowicz J.R., Geddes C.D., Gryczynski I., Malicka J., Gryczynski Z., Aslan K., Lukomska J., Matveeva E., Zhang J., Badugu R., Huang J. (2004) J Fluoresc 14(4): 425 -41.
  6. Metal-enhanced fluorescend: an emerging tool in biotechnology Aslan K, Gryczynski I, Malicka J, Matveeva E, Lakowicz JR, Geddes CD. Curr Opin Biotechnol. (2005) 16(1):55-62.
  7. Plasmonic technology: novel approach to ultrasensitive immunoassays. Lakowicz JR, Malicka, J., Matveeva E, Gryczynski I, Gryczynski Z. Clin Chem (2005) 51: 1914-22
  8. Weisenberg, M, Zhang, Y. and Geddes, C. D., (2010). Metal-Enhanced Chemiluminescence from Chromium, Copper, Nickel and Zinc Nanodeposits: Evidence for a second enhancement mechansim in metal-enhanced fluorescence. Applied Physics Letters, 97: 133103.
  9. Dragan, A. I., Bishop, E. S., Casas-Finet, J. R., Strouse, R. J., Schenerman, M. A., and Geddes, C. .D, (2010). Metal-Enhanced Picogreen Fluorescence: Application to fast and ultra-sensitive pg/ml DNA quantitation, Journal of Immunological Methods, 362, 95-100.
  10. Large Fluorescence Enhancements of Fluorophore Ensembles with Multilayer Plasmonic Substrates: Comparison of Theory and Experimental Results. Szmacinski H, Badugu R, Mahdavi F, Blair S, Lakowicz JR. J Phys Chem C Nanomater Interfaces (2012) 11;116(40):21563-21571.
  11. Imaging of Protein Secretion from a Single Cell Using Plasmonic Substrates. Szmacinski H., Toshchakov V., Piao W., Lakowicz J.R. Bionanoscience. (2013) 1;3(1):30-36.
  12. Tennant SM, Zhang Y, Galen JE, Geddes CD, Levine MM. (2011) Ultra-fast and sensitive detection of non-typhoidal Salmonella using microwave-accelerated metal-enhanced fluorescence (NAMEF). PLoS One. 8;6(4):e18700.
  13. Melendez, J.H., Huppert, J.S., Jett-Goheen, M., Hesse, E.A., Quinn, N., Gaydos, C.A. and Geddes, CD. (2013). Blind Evaluation of the Microwave-Accelerated Metal-Enhanced Fluorescence Ultrarapid and Sensitive Chlamydia Trachomatis test by use of Clinical Samples, Journal of Clinical Microbiology, 51(9), 2913-2920.
  14. Zhang, B., Kumar, R.B., Dai, H., and Feldman, B.J.  (2014)  A plasmonic chip for biomarker discovery and diagnosis of type 1 diabetes. Nature Medicine  20: 948–953.

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