Pressure-induced Spectral Shifts Research Articles

Multimodal, super-sensitive luminescent manometer based on giant pressure-induced spectral shift of Cr3+ in the NIR range

The remote pressure readout is of vital application importance as it will facilitate and in many cases enable rapid diagnostics. For most of the luminescent manometers described to date, the sensitivity of the pressure readout is limited by the relatively small pressure-induced spectral shift of the emission band. In response to this limitation, in this work, we present a super-sensitive and temperature-invariant NIR emitting luminescent manometer based on LiScGeO4:Cr3+. The 4T2→4A2 emission band of Cr3+ ions undergoes in this manometer a linear spectral shift with a rate of dλ/dp = 23.63 nm/GPa, which is the highest value reported in the literature to date. In addition, by using a ratiometric approach, this manometer has a relative sensitivity exceeding 120%/GPa. Importantly, the developed optical manometer operates in the NIR spectral region, providing good light permeability in the non-transparent systems, having high absorption in the visible range. Moreover, in the case of high-pressure luminescence experiments, the use of the developed sensor will eliminate spectral overlapping issues, as the vast majority of the organic and inorganic phosphors emitting in the visible range.

Temperature invariant ratiometric luminescence manometer based on Cr3+ ions emission

The dependence of the spectroscopic properties of the phosphor on the applied pressure enables the development of luminescent manomenters that allow remote readout and imaging of the pressure. Although the most widely used manometers for years have been those using pressure-induced spectral shift of the emission band, their main limitations are relatively low sensitivity for spectrally narrow emission lines or low readout precision for broadband phosphors. Taking advantage of the benefits of both these approaches, this paper proposes a ratiometric pressure-readout strategy that exploits spectral shift of the 4T2→4A2 emission band of Cr3+ ions in Li2Mg3TiO6. The luminescent manometer developed in this way is characterized by a linear dependence of LIR on pressure in the range of 0–7.75 GPa pressure range and a relative sensitivity of SR = 4.7 %/GPa. Importantly, this manometer is characterized by insensitivity to temperature changes in the 123–563 K. This novel approach will allow the development of a new group of highly sensitive luminescent manometers.

Pressure-Induced Spectral Shifts in GFP Mutants Explained by Molecular Dynamics Simulations

By combining spectroscopic measurements under high pressure with molecular dynamics simulations we investigate how sub-angstrom structural perturbations are able to tune protein function. We monitored the variations in fluorescence output of two Green Fluorescent Protein mutants (termed Mut2 and Mut2Y, the latter containing the key T203Y mutation) subjected to pressures up to 600 MPa, at various temperatures in the 280-320 K range. By performing ∼100ns molecular dynamics simulations of the protein structures at various pressures, we evidenced subtle changes in conformation and dynamics around the light-absorbing chromophore. Such changes explain the measured spectral tuning, especially in the case of the sizable 120 cm-1 red-shift observed for pressurized Mut2Y, but absent in Mut2. Previous work by Barstow et al [1] on pressure effects on GFP also involved a T203Y mutant. Using cryocooling X-ray crystallography at high pressure they linked the observed, pressure-induced, fluorescence blue shift at low temperature (77 K) to key changes in relative conformation of the chromophore and Y203 phenol ring. At room temperature, however, they observe a red shift at high pressure, analogous to the one in Mut2Y. Our investigation of structural variations in compressed Mut2Y also explains their result, bridging the gap between low-temperature and room-temperature high-pressure effects.[1] Barstow, B., Ando, N., Kim, C. U., & Gruner, S. M. (2008). Alteration of citrine structure by hydrostatic pressure explains the accompanying spectral shift. Proc Natl Acad Sci USA, 105(36), 13362-13366.

Protein elasticity determined by pressure tuning of the tyrosine residue of ubiquitin

We determined the isotropic, isothermal compressibility of ubiquitin by pressure tuning spectral holes burnt into the red edge of the absorption spectrum of the single tyrosine residue. The pressure shift is perfectly linear with burn frequency. From these data, a compressibility of 0.086 GPa(-1) in the local environment of the tyrosine residue could be determined. This value fits nicely into the range known for proteins. Although the elastic behavior at low temperatures does not show any unusual features, the pressure tuning behavior at room temperature is quite surprising: the pressure-induced spectral shift is close to zero, even up to very high pressure levels of 0.88 GPa, well beyond the denaturation point. The reason for this behavior is attributed to equally strong blue as well as red spectral pressure shifts resulting in an average pressure-induced solvent shift that is close to zero.

Influence of ligand spin–orbit coupling to the sign of the zero-field splitting and pressure-induced spectral shift for ZnS:Mn2+ (bulk/nanocrystal)

Abstract Serious difficulties exist in explaining the zero-field splitting (ZFS) of 3d 5 ions in crystal, with the current crystal-field theory. The calculated cubic ZFS a -value of 3d 5 ion is positive identically. However, K.A. Muller and W. Low found experimentally that a is negative for some ZnS:Mn 2+ crystals. In this work, an unified explanation is developed for the ZFS, optical spectra and pressure-induced spectral shift for the ZnS:Mn 2+ (bulk/nanocrystal) by considering the influence of the spin–orbit coupling to the ZFS and spectral bands. The excellent agreement between calculation and experiments shows that the above-mentioned difficulties can be removed based on the calculation model proposed by authors. Calculation result shows that there are two kinds of stable electron states with ( λ π , λ σ , λ s ) = (0.2713448, −0.1619936, −0.08) and (0.2713448, 0.346885, −0.220), respectively, where ( λ π , λ σ , λ s ) denote the mixing coefficients of Mn 2+ − 4S 2− anti-bonding in ZnS:Mn 2+ .

High-Pressure Raman Study of Anthracene

Pressure-induced Raman shifts and line broadening in crystalline anthracene have been studied up to 3.1 GPa at ambient temperature. The use of nitrogen as a hydrostatic pressure medium eliminated pressure-induced defect fluorescence that had previously restricted high-pressure Raman studies of anthracene. Mode Grüneisen parameters have been determined for six phonons and nine vibrons, and some evidence for a previously suggested phase transition at 2.4 ± 0.2 GPa is observed in the line width data. The calibrated pressure-induced spectral shifts and line width changes also can be utilized as a gauge of the pressure and temperature profile of a laser-induced shock front (Hambir, S. A.; Franken, J.; Hare, D. E.; Chronister, E. L.; Baer, B. J.; Dlott, D. D. J. Appl. Phys. 1997, 81, 2157).

Effect of hydrostatic pressure on the fluorescence of indole derivatives

Effects of hydrostatic pressure on the fluorescence emission of L-tryptophan, N-acetyl-L-trytophanamide and indole were investigated. An increase in pressure ranging from 1 bar to 2.4 kbar results in reversible red-shifts of the emission of the three fluorophores. The pressure-induced redshift amounts to about 170 cm(-1) at 2.4 kbar, and appears related to changes in Stokes shift of the fluorophores caused by pressure effects on the dielectric constant and/or refractive index of the medium. As the pressure range investigated here is the range commonly used in studies of protein subunit association and/or folding, these observations raise the need for caution in interpreting pressure-induced spectral shifts. The significance of these observations to pressure studies of proteins is illustrated by investigation of pressure effects on human Cu,Zn Superoxide dismutase (SOD) and azurin fromPseudomonas aeruginosa. A reversible 170 cm(-1) red-shift of the emission of SOD was observed upon pressurization to 2.4 kbar. This might be interpreted as pressure-induced conformational changes of the protein. However, further studies using SOD that had been fully unfolded by guanidine hydrochloride, and fluorescence anisotropy measurements indicated that the observed red-shift was likely due to a direct effect of pressure on the fluorescence of the single tryptophan residue of SOD. Similar pressure-induced red-shifts were also observed for the buried tryptophan residue of azurin or for azurin that had been previously denatured by guanidine hydrochloride. These observations further suggest that the effective dielectric constant of the protein matrix is affected by pressure similarly to water.

Estimation of the local compressibilities for GD3Ga5O12 : Cr3+ and Gd3Sc2Ga3O12 : Cr3+ crystals from their high pressure spectroscopies

According to high pressure spectroscopies, in particular, the nearly linear red-shift of the R-line at pressures above approximate to 7 GPa for Gd3Ga5O12:Cr3+ and Gd3SC2Ga3O12:Cr3+ crystals, the local linear compressibilities of (CrO6)(9-) groups in both systems at P approximate to 7 GPa have been determined by using the bond length dependence of the covalency reduction factor obtained from the pressure-induced spectral shifts of ruby (Al2O3:Cr3+). Then, the local compressibilities at normal pressure are estimated on the basis of the pressure dependence of compressibility obtained for other garnet crystals. The results are obviously different from the compressibilities of the host crystals. The reasons of this difference are discussed.

Local compressibility around the impurity in nickel-doped CaO crystal

In this paper, by using the bond length dependence of Racah parameter B obtained from the high pressure spectroscopy of MgO: Ni 2+ crystal, the local compressibility around the impurity ion in nickel-doped CaO crystal has been determined from the pressure-induced spectral shifts. The result is greater than that in the pure CaO crystal. The reasonableness of this local value is discussed.

Determination of the local compressibilities for Cr3+ ions in some garnet crystals from high-pressure spectroscopy

By using the bond length dependence of the covalency reduction factor obtained from the pressure-induced spectral shifts of ruby (Al2O3:Cr3+), the local compressibilities of Cr3+-O2- bonds for Cr3+ in the octahedral a sites of Y3Al5O12 and La3Lu2Ga3O12 crystals have been determined from their high-pressure spectroscopies. The results are obviously different from the compressibilities of the host crystals. The reasons for the difference are discussed.

Various contributions to the thermal shift of the R-line of MgO:V2+

The electron-phonon interaction causes a spectral thermal shift (TS) in crystals with impurities. In this paper we develop an approach to calculate the different contributions to the TS of MgO:V2+ in detail; these include Raman scattering, the direct process and the optical branch. It is found that the direct process plays a predominant role in the TS. Moreover, the TS caused by thermal expansion is interpreted and calculated using the theory of the pressure-induced spectral shift. Taking account of the contributions, we calculated the R-line TS of MgO:V2+ by fitting the experimental data. Our results show good agreement with the TS experimental data and other experimental data for MgO:V2+.

Thermal Shift Calculations (I): The Theory for Thermal Shift of Crystal Field Spectra of Rare Earth or Transition Metal Ions

The thermal shift of crystal field spectra of rare earth or transition metal ions includes four contributions of different characteristics. In this paper we develop a theory of calculating all those contributions in detail. The contributions to thermal shift consist of that of thermal expansion and those of electron-phonon interaction. The thermal shift caused by thermal expansion is interpreted and calculated by making use of theory of pressure-induced spectral shift. The contribution of phonons of optical branches is given by using single frequency model. The contributions of phonons of acoustic branches include two terms, which are derived by taking into account contributions of all the in electron-phonon interaction. The theoretical formulas for parameters in the most important term are given.

Single molecule spectroscopy: Stark-, pressure-, polarization-effects and fluorescence lifetime measurements

Properties of single molecules at low temperatures can be studied by fluorescence excitation spectroscopy. Stark effect, pressure-induced spectral shifts, polarization effects, and time-resolved measurements have been performed on single pentacene molecules embedded in a p-terphenyl crystal at low temperature.

Theory of spectral thermal shift and R-line thermal-shift calculations of MgO:Cr3+

The thermal shift of the crystal-field spectra of 3d or 4f ions includes contributions of different characters. In this paper we develop an approach to calculating all those contributions in detail. The thermal shift caused by thermal expansion is interpreted and calculated by use of the theory of pressure-induced spectral shift. Contributions of phonons of optical branches are given using a single frequency model. Taking into account contributions of all the \ensuremath{\Gamma}\ensuremath{\gamma} in the electron-phonon interaction, two terms coming from phonons of acoustic branches are derived in detail, and theoretical formulas for the parameters in the most important term are given. By adequately taking into account all the contributions, we calculated the R-line thermal shift of MgO:${\mathrm{Cr}}^{3+}$ by fitting the experimental data. The results show good agreement with thermal-shift experimental data and a series of other experimental data of MgO:${\mathrm{Cr}}^{3+}$. Values of the most important parameters are calculated theoretically with wave functions obtained from diagonalization of complete ${\mathit{d}}^{3}$ energy matrices. They are in good agreement with those from fitting the experimental data. We found that it is the mixing of wave functions caused by the Coulomb interaction and/or the spin-orbital interaction that makes D(${\mathrm{\ensuremath{\Gamma}}}_{6}$) and D(${\mathrm{\ensuremath{\Gamma}}}_{8}$) nonzero. Our investigation shows that among all the contributions to thermal shifts, the ${\mathit{t}}_{2}^{32}$${\mathit{T}}_{1}$ term (resulting in redshift) is the most important one; the Raman term (resulting in blueshift) eliminates part of the ${\mathit{t}}_{2}^{32}$${\mathit{T}}_{1}$ term; their algebraic sum gives the contribution of electron-phonon interaction of the acoustic branches; contributions of optical branches (resulting in redshift) increases rapidly with temperature, and exceeds that of the acoustic branches when T\ensuremath{\ge}175 K; contribution by thermal expansion (resulting in blueshift) holds about 30% of total thermal shift in absolute value.

D-orbital theory for an octahedral-site-symmetry crystal and pressure-induced spectral shifts in ruby.

A d orbital for the transition-metal ion in an octahedral-site-symmetry crystal is proposed by considering the ligand-charge penetration. The expanded radial d-orbital wave function in the crystal is expressed as a function of the bond length. With this d orbital the nephelauxetic decrease of Racah parameters B, C, spin-orbit-coupling parameter ξ d , and their pressure-induced reductions can be described quantitatively with a simple point-charge model. The theory is applied to calculate the d-d transitions (R, R', Y, B, and U) in ruby and their pressure-induced shifts. The calculated results are in good agreement with experimental data

The Pressure-Induced Spectral Shift Calculations for Seven Crystals with MgO, CaO, or α-Al2O3 as the Hosts11Projects partly supported by the National Natural Science Foundation of China.

The conception of expansion function ϕ(κ) at the exponential of d electron radial wavefunction of transition-metal cations developed in Refs. [1–5] was used to interpret pressure-induced spectral shifts of series of crystals. The red or blue shifts of the spectra of 4 crystals with MgO or CaO as hosts and 3 crystals with α-Al2O3 as hosts were calculated by making use of the parameters obtained from fitting the experimental spectral shifts of MgO:Cr3+ and α-Al2O3:Cr3+ crystals respectively. The results are satisfactory, and it has been found that ϕ(κ) and η(κ) are mainly effected by the host while the difference of the impurity transition-metal ions causes ϕ(κ) and η(κ) varied only slightly.

Crystal Spectra of Tetracene and Rubrene under Pressure

Pressure-induced spectral shifts (0–20 kbar) are compared for the absorption and fluorescence of tetracene and rubrene as crystals or embedded in polystyrene matrices. At 20 kbar the red shift of the tetracene crystal absorption is two to three times larger than either the red shift of tetracene in polystyrene or rubrene in both crystal and solid-solution environments. The crystal shifts and increased Davydov splitting at high pressure are related to calculations on compressed anthracene by Rice and Jortner. The spectral shifts indicate that crystal-field effects are negligible for rubrene due to steric factors. Strong specific interactions in the excited state between the phenyl substituents of rubrene and the phenyl groups of polystyrene are indicated from the fluorescence spectra at atmospheric pressure. The influence of pressure on tetracene crystal luminescence can be described in terms of excitation-energy transfer to tetracene dimers at defects. High pressures enhance the transformation of excitation energy into heat, probably by increasing non-radiative excition trapping at imperfections.

Pressure-induced spectral shifts in hemoproteins

Pressure-induced spectral shifts in hemoproteins