Fluorescence imaging induces DNA damage

Biological molecules thought imaged by UV fluorescence from surface plasmons are in fact excited by fluorescence from higher energy EUV radiation that damages DNA
By: QED Radiations
Fluorescent imaging: LSPR materials and UV sensitivity
Fluorescent imaging: LSPR materials and UV sensitivity
PITTSBURGH - Aug. 8, 2017 - PRLog -- .

Plasmonics is known [1] to hold tremendous potential for NSs of Au and Ag having LSPR at IR and VIS frequencies, the LSPRs exciting nearby molecules by fluorescence imaging. NS stands for nanostructure and LSPR for local surface plasmon resonance. However, extending imaging to proteins that are fluorescent in the UV is not thought possible because Au and Ag LSPRs are limited to wavelengths > 550 nm and > 350 nm, respectively. In this regard, Al is proposed as an alternative plasmonic material in the UV > 200 nm.

Molecules excited by UV radiation may emit VIS radiation. But UV fluorescence differs as VIS is emitted not upon UV excitation, but rather by even shorter wavelength EUV radiation, the excited states of which do not directly return to the ground state, but rather pass through many excited states that cause the VIS emission. EUV stands for extreme UV < 200 nm.

Today, LSPRs in the VIS are unfortunately thought to be the only source of molecular fluorescence from metallic NSs. Other EUV excited states of NSs are ignored as evidenced by the fact that in the absence of external EUV sources, molecular fluorescence is observed near metal NSs suggesting the EUV is somehow being produced within the NS itself. To date, EUV from NSs has confounded the interpretation [1] of enhanced fluorescence of nanoscale Al films under external EUV.

In this regard, the thumbnail shows the peak wavelengths of extinction efficiency [2] for Al and Au to be 377 and 665 nm suggesting Al is better than Au for molecular fluorescence in the UV, although the data for Au < 400 nm is not presented, i.e., Al only appears better than Au in the VIS. In the EUV < 200 nm, NSs of Au are far more absorbent than in the VIS and therefore Al may not be more fluorescent than Au and Ag.

Simulations of LSPRs is based on Mie theory [2, 3] that assumes NS do not alter the frequency of EM radiation upon absorption. But QM governs the nanoscale and not classical physics. QM stands for quantum mechanics. Because NSs have high S/V ratios, EM radiation is absorbed in the NS surface thereby placing interior atoms under high EM confinement that by the Planck law of QM precludes atoms from having the heat capacity necessary to conserve surface heat by an increase NS temperature. S/V stands for surface-to-volume.

Conservation of heat then proceeds by simple QED creating EM radiation having half-wavelength λ/2 = d standing across the dimension d of the NS. The complex relativistic QED advanced by Feynman is not required. Briefly stated, simple QED creates EM radiation from surface heat having Planck energy E = hν. Here, h is Planck's constant and ν is frequency, ν = (c/n) / λ = c/2nd, where the velocity of light c is corrected for the slower speed in the solid state by the refractive index n of the NS. Unlike Mie theory, the frequency ν may be red or blue shifted depending on the dimension d of the NS. But once NS surface heat is expended in forming the standing waves, the EM confinement vanishes and the QED induced EM radiation escapes into the surroundings. See diverse simple QED applications of the Planck law at http://www.nanoqed.org/, 2010 – 2017.

In SPCE, a fluorescent molecule [3] near an aluminum film on a glass substrate is assumed to excite surface plasmons in the film which enhance the fluorescence upon radiating back into the glass. SPCE stands for surface plasmon-coupled emission. The enhancement is 11 fold compared to the bare glass, but the EUV source of the emission is not clear. In contrast, QED induced EM radiation based on the EUV source interprets the enhancement of NS emission in as a QM effect. For a spherical Al NS of diameter d = 20 nm, the EUV created at wavelength λ = 2nd has Planck energy E= hc/λ. In the EUV, the Al refractive index n is < 1, but since the speed of light cannot be > c, n =1. Hence, the EUV wavelength λ = 2nd = 40 nm and E = 31 eV. What this means is the extinction peak at 150 nm is excited by fluorescence from simple QED induced EUV radiation at 40 nm produced in the NS upon conserving thermal heat from the surroundings.

Fluorescence microscopy [4] is commonly used for imaging live mammalian cells, but there are associated genotoxic effects with standard bio-imaging. To assess DNA damage, light emitted at violet [340–380 nm], blue [460–500 nm], and green [528–553 nm] wavelengths all showed exposure during imaging to be genotoxic. Green expressing proteins showed DNA damage even in nearby cells not directly imaged. DNA damage from shorter wavelength EUV [100-200 nm] common to Deep-UV LSPR enhancements [1-3] cited here were not tested, but are expected to be far more severe.

QM by the Planck law precludes heat or absorbed external UV radiation from being conserved by increasing the NS temperature. Instead, conservation proceeds by simple QED producing EM radiation that induces fluorescence.

Classsical Mie theory that assumes EM frequencies do not change upon absorption in NS is superseded by QM allowing EM radiation to red or blue shift  depending on NS dimensions.

Fluorescent bio-imaging damages DNA and exposures should be limited.

[1] M. W. Knight, et al., "Aluminum for Plasmonics," ACS Nano, 8 , 1 , 834-840, 2014.
[2] G. H. Chan, et al., "Localized Surface Plasmon Resonance Spectroscopy of Triangular Aluminum Nanoparticles," J. Phys. Chem. C,112,13958–13963, 2008.
[3] M. H. Chowdhury, et al., "Aluminum Nanoparticles as Substrates for Metal-Enhanced Fluorescence in the Ultraviolet for the Label-Free Detection of Biomolecules," Anal. Chem. 81, 1397–1403, 2009.
[4] J. Ge, et al., "Standard Fluorescent Imaging of Live Cells is Highly Genotoxic," Cytometry A., 83,552–560, 2013.

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