Nano-thermometers and Quantum Mechanics

Sept. 27, 2011 - PRLog -- Background

Chemical reactions inside a cell normally occur only at much higher temperatures than body temperature. Metabolic reactions therefore utilize enzymes to increase chemical reactivity that controls cell temperature. Cell temperature itself is inconsequential compared to enzymes in driving chemical reactions. Hence, the notion that temperature is one of the most important physical factors in a chemical reaction inside a cell can safely be dismissed. Nevertheless, temperatures within living cells may be important in understanding how enzymes produce almost constant body temperatures.

Scientists have recently reported the measurement of temperatures inside mouse cells using nano-thermometers, called QDs. QD stands for quantum dot. See http://www.tempsensornews.com/contact/temps-for-individual-cells/. QDs are sub-micron nanoparticles (NPs) of semiconductor materials that are small enough to enter individual cells. Temperature measurements [1-3] are made by exciting the QDs with a laser to obtain the QD emission spectra, the QD temperatures inferred from experimental QD spectra taken at known temperatures.  

The measurement of cell temperatures based on laser excited QD emission spectra has been proposed before. Indeed, a primer [2] on the design of nanoscale thermometers has led to many applications, including [3]. The QD emission spectrum depends on photons, but phonon confinement mechanisms [4-6] are also proposed using NPs to measure cell temperatures.  Raman shifts of laser excited NPs based on phonon confinement are used to measure cell temperature from experimental correlations of NP size with temperature dependent grain growth.  

Problems

QM requires the heat capacity of QDs and NPs to vanish, and therefore any laser excitation cannot be conserved by an increase in temperature. What this means is QD emission spectra or Raman shift in NPs may have nothing to do with their temperatures. A clarification of what the QDs and NPs are actually measuring is required.

QED Induced Radiation

By the theory of QED induced radiation, the observed VIS fluorescence and IR Raman shifts are a consequence of absorbed EM energy that includes not only metabolic heat, but also the light from the respective laser excitation. QED stands for quantum electrodynamics and EM for electromagnetic. Since absorbed EM energy in a QD or NP is not conserved by an increase in temperature, conservation proceeds by the frequency up-conversion of the absorbed EM energy to their TIR confinement frequencies. TIR stands for total internal reflection. TIR is enhanced by the fact QDs and NPs having a high surface to volume ratios concentrate the absorbed EM energy in their surface thereby providing the confinement necessary to create the high-energy QED photons. Subsequently, the QED photons as the EM source excite the lower energy VIS fluorescence states and induce Raman shifts. All this occurs without an increase in QD or NP temperature. See forthcoming Paper in http://www.nanoqed.org at “Nano-thermometers,” 2011.    

Discussion

Intra-cellular Temperature Gradients The claim [1] that QDs show temperature gradients develop in cells under metabolic heat requires qualification. In fact, temperatures measured from the QD spectra by exciting the QDs with laser light have nothing to do with the temperature of the QDs themselves, as QM precludes any temperature increase upon the absorption of laser light. However, the QDs in contact with the cell become part of a macroscopic thermal sink, thereby acquiring the temperature of the cell without altering its temperature upon laser excitation.

What this means is the measurements of cell temperatures [1] are only valid if the QDs are in contact with or fixed to the cell during laser excitation. If the QDs move, contact is lost and the laser excitation is converted to QED photons that excite VIS fluorescence independent of the cell temperature. Indeed, the difference in measured temperatures by QDs that are fixed and moving is observed in (Fig. 2d and inset of [1]). By QM, only the fixed QD response is representative of the cell temperature. In the thumbnail, the bright red images are most likely moving QDs not related to cell temperature.

Temperatures in QD Emissions The 18 nm QDs are assumed [3] capable of up-converting laser light at 980 nm to higher energy green light at 515 and 535 nm. A 2-photon process of energy transfer from Yb3+ to Er4+ is proposed to explain the up-conversion. However, the 2-photon process cannot explain the UV absorption typical of QDs that far exceeds that at the plasmon resonances in the VIS. Instead, the QD spectrum is proposed caused by the creation of QED photons at energies beyond the UV that act as the EM source to excite the fluorescent VIS states. Provided the QDs are in contact with the cell, the temperature of the QD inferred from QD spectra is a valid determination of cell temperatures.  However, it is not known if the QDs in (Fig. 1B of [3]) were or were not in contact with the cuvette during calibration of the QD spectra.      

Cancer and QD Temperatures Cancer cells are typically 10-20 microns and by QM are macroscopic allowing temperatures to increase under metabolic heating. In contrast, QDs are submicron and under heating cannot increase in temperature. Provided the QDs remain in contact with the cancer cell, the cell temperature measurement [3] based on QD spectra is valid, but if the QDs come off the cell, the absorbed laser excitation is emitted as fluorescent QED radiation having nothing to do with the temperature of the cell.

Phonon Confinement Unlike the physical basis for the TIR confinement of photons, phonon confinement is phenomenological. But TIR confinement in NPs also occurs upon laser excitation in Raman shift measurements. Although phonon confinement as a theory is sometimes [6] questioned, the correlation of Raman shifts with the growth of NPs at temperature over a time is valid for a specific type of NP.                

Conclusions

1. Provided the QDs are in contact or fixed to the cell, QM allows them to acquire the cell temperature. Analysis of the QD spectrum obtained by exciting fixed QDs with a laser therefore provides a valid estimate of cell temperature, i.e., the cell temperature varies less than about 1 C.

2. However, if the QDs move and lose contact with the cell, QM precludes any temperature response of QDs under laser excitation, and instead QED photons are created inside the QDs that excite VIS fluorescent modes giving false cell temperature measurements, i.e., temperature differences up to 10 C.  

3. The hypothesis that cells use differences in temperature as a way to communicate is unlikely because fixed cell temperatures are less than 1 C. Instead, cells may use natural QDs of attached sub-micron proteins to communicate their temperature by EM signaling using QED induced EM radiation.

4. The validity of NPs in phonon confinement to measure cell temperature is limited by QM for the same reasons described above for temperatures measured using QD spectra.    

References

1. J-M Yang, et al., “Quantum Dot Nano Thermometers Reveal Heterogeneous Local Thermogenesis in Living Cells,” AC Nano, 5, 567 (2011)
2. J. Lee and N. A. Kotov, “Thermometer Design at the Nanoscale,” Nano Today, 2, 48-51 (2007).
3. F. Vetrone, et al., “Temperature Sensing Using Fluorescent Nanothermometers,” AC Nano, 4, 3254–3258 (2010).
4.  J.Wang and L. Huang, “Thermometry based on phonon confinement effect in nanoparticles,” Appl. Phys. Let., 98, 113102 (2011)
5.  V, Swamy, et al., “Nonlinear size dependence of anatase TiO2 lattice parameters,” Appl. Phys. Let., 88, 243103 (2006)
6.  T. Mazza, “Raman spectroscopy characterization of TiO2 rutile nanocrystals.” Phys. Rev.B, 75, 045416 (2007)
.

# # #

About QED Induced EM Radiation: Classically, absorbed EM energy is conserved by an increase in temperature. But at the nanoscale, temperature increases are forbidden by quantum mechanics. QED radiation explains how absorbed EM energy is conserved at the nanoscale by the creation of nonthermal EM radiation.