Silica fluorescence spectra

Figure 8.2 presents the fluorescence of pyrene on silica gel. The loading is low so that pyrene is predominantly adsorbed as nonaggregated monomers (Mi). The backward fluorescence spectrum Fb of this sample is very comparable to the spectrum in polar solvents and not distorted by reabsorption. However, the forward spectrum Ft is almost completely suppressed in the region of overlap with the o -transition and hot sidebands of the weak first absorption band Si. The absorption coefficients of the sample vary widely from k" = 0.1 cm 1 (Si-band, Aa = 350-370 nm) to k = 25 cm-1 (S2-band, 1 290-340 nm), and in a first approximation the excitation spectrum of Fh reflects this variation correctly (Figure 8.2, left). The Ff-excitation spectrum, however, has only little in common with the real absorption spectrum of the sample. 

Fig. 2.12. (a) Fluorescence spectrum of sennoside A on a silica gel TLC plate after spraying with hydrazine. Detection Aminco-Bowman spectrofluorimeter, with a TLC-scanning accessory. Ex = excitation, Em = emission, (b), Scan of a TLC analysis of a Sennokot tablet extract obtained with a Zeiss chromatogram scanner. Peaks 1 3 sennoside B 2 = sennoside A 3 = sennoside C. 

Methylcarbamate insecticides have been recently labeled with DNS-C1 [145]. The procedure involves the hydrolysis of the carbamates with 0.1 M sodium carbonate to form a phenol and methylamine [166]. The two hydrolysis products are labeled with DNS-C1 and subsequently detected and determined quantitatively by TLC on silica gel layers by scanning spectrofluorimetry in situ. The reaction conditions were examined, and optimum conditions for hydrolysis and labeling were established [167]. The overall reaction scheme is shown in Fig. 4.62. The phenol derivatives of a number of N-methylcarbamates have been separated by one- and two-dimensional TLC [168], and the fluorescence behaviour and stability of the derivatives have been examined [169]. Most of the DNS derivatives fluoresce at similar wavelengths (excitation, ca. 365 nm emission, ca. 520 nm). The fluorescence spectrum of a typical DNS derivative is shown in Fig. 4.63. The method has been applied successfully to the analysis of low concentrations of carbamates in water and in soil samples with little or no clean-up being required [170,171]. Amounts as low as 1 ng of insecticide can be detected instrumentally. Visual limits of detection are ca. 5-10 ng per spot. 

Spectroscopy. It has been shown previoiasly L that the fluorescence spectrum of Ruthenim tris-bipyridine, RuII, is solvent dependent, showing a red shift with increasing solvent polarity. The fluorescence spectrvnn of RuII on silica particles is identicel to that of the excited molecule in water. It will be shown subsequently that the RuII is essentially all botmd to the silica particle, hence the data show that the environment of a probe molec ile such as RuII on silica particles is very polar and similar to water. 

Di(1-pyieny1)propane. Bauer et al. (32) observed an excimer-like emission in the fluorescence spectrum of 1Py(3)1Py on dry silica, which was attributed to intramolecular interactions in the ground state. The excimer emission disappeared on adsorption of 1-decanol to the surface. In contrast, strong intramolecular excimer fluorescence was found by Avnir et al. for 1Py(3)1Py adsorbed on a silica surface with up to a double-layer equivalent of 1-octanol (38), see Section 4.3.2. Excimer formation was also detected on reversed-phase Si-C g and on untreated silica, the ratio I /I depending on the amount of 1Py(3)1Py adsorbed (38), see Fig. 3. 

From the vibrational structure in the monomer fluorescence spectrum of Py, the Ham-effect (54-58), information was obtained on the polarity of the surface where the excimer and dimer formation takes place. It appears that Py on silica/decanol resides in an environment similar to a homogeneous decanol solution (36), whereas for Py on dry silica a considerably higher polarity was found between that of methanol and water, see Fig. 8. For rever-sed-phase Si-C.jg in contact with methanol/water 3/1 (Section 4.2.3), it was concluded that Py finds itself in a nonpolar medium (21). 

Photophysics of 1-Aminopyrene on Various Silica Gel Surfaces. Figure 2 presents the fluorescence spectra of 1-AP adsorbed on MCB and FS-662 silica gel in cyclohexane. The fluorescence spectrum of MCB-bound 1-AP is quite typical of 1-APH+, whereas the fluorescence spectrum of FS-662-bound 1-AP is quite typical of 1-AP. The surface-bound 1-APH+ decays with an inherent unimolecular lifetime of 135 2 ns, and the surface-bound 1-AP decays with an inherent unimolecular lifetime of 4.9 0.1 ns. The different photophysical behavior of the 1-AP indicates that the adsorption sites for 1-AP are different on these silica gel samples at the given probe loadings. 

Moderate heat treatment of the MCB silica gel at 450 °C for 24 h, a temperature below the sintering temperature, alters the surface such that the fluorescence spectra of 1-AP adsorbed on this surface have characteristics of both 1-AP and 1-APH+ a heating temperature of 650 °C yielded an adsorbed 1-AP spectrum identical to that of 1-AP adsorbed on FS-662. A several-hour concentrated nitric acid treatment of the FS-662 silica, followed by washing with water until the wash maintained a neutral pH and drying, alters the surface such that the fluorescence spectrum of 1-AP adsorbed to this surface resembles that obtained from the MCB surface. 

The fluorescence spectrum of pyrene has been very often nsed as a photophysical probe. The fluorescence spectmm shows the vibronic strnctnre, which is sensitive to the polarity of the environment (Nakajima, 1971 Kalyanasnndaram, 1977). Moreover, pyrene forms excimer due to one molecnle in the groimd state and one in the excited state for concentrated systems. Therefore, fluorescence spectra of pyrene can be expected to reveal the stmctnral changes of sol-gel silica and trapping mechanism of organic molecnles along the sol-gel-xerogel transitions... 

Further, in cases where the background fluorescence cannot be entirely eliminated, the Raman spectrum of an adsorbed species appears as a superimposed signal. Figure 2B shows the Raman spectrum of a methylated silica, a sample where all surface Si OH groups have been replaced by SiOCH3 groups. The sharp features near 3000 cm-I due to v(CH) modes will be discussed further later, but the signal to fluorescence intensity can be estimated from the ordinate scale. 

The similarities are obvious. In all cases, excitation maximum is at 356 nm and emission at about 422 nm. The blank shown is the spectrum from a polyamide plate exposed in a similar situation but without lipid. It shows the residual of the scatter peak at 360 nm which is not removed by the 39 filter. There is also a pattern of diffraction peaks produced by the polyamide (and, indeed, by any fine powder-coated surface like silica TLC plates). The wavelengths of this diffraction pattern are excitation wave-length-dependent, unlike the situation in normal fluorescence, a given peak of which is conservative in wavelength with changes in... 

The fluorescence excitation spectrum of acridine in O.IN sulfuric acid has a band characteristic of acridinium cation at about 25000 cm, but on the other hand, acridine in ethanol has no band below 25000 cm" (see Fig. 8). The fluorescence excitation spectra of acridine both on SG200V and SG200A have a band near 25000 cra . These findings indicate that a proton is transferred to acridone from silanol groups in its ground state as a result of adsorption on the silica gel surface. The emission of acridine adsorbed on silica gel are reasonably assigned to the protonated species of acridine. 

Emissions from both the and the previously unreported lli states of the IF molecule have been observed in the gas-phase reaction of L with F2 at low pressure a four-centre complex has been proposed as the reaction intermediate. A combined theoretical-experimental programme has been conducted to establish techniques for the study of excited-state transitions in Ij and IC1. Experimental techniques based on two-step excitation using two synchronized, tunable lasers have been developed, and successfully applied to excited-state fluorescence measurements on ICl. lodine(i) chloride adsorbed on silica gives the same Raman spectrum as that obtained from adsorbed l2. ... 

Upon adsorption onto colloidal silica, the monomer excitation spectrum remained essentially unchanged from that of the solution. In this system, the excimer persisted, allowing study of its excitation spectrum. In this experiment, a substantial shift in the excimer excitation spectrum occurs, indicating that, although excimer-forming sites remain, they exist in a strained state. This observation is further supported by the time dependence of the excimer fluorescence decay. Whereas chains remain adsorbed on the colloidal silica surface, the excimers formed are substantially shorter lived than those in free solution. 

This correlation is illustrated in Table I, which shows the fluorescence lifetime and frequency shift for the excimer excitation spectrum relative to that of the free solution. The first six values correspond to different polymer solution concentrations ranging from [Py-PEG-Py(8650)] = I x 10" to 4 X 10 M with added untagged PEG(22,000) of 0 and I x I0 M. In this regime, the tagged chains remain on the silica particle surfaces. All these... 

Hexavalent uranium fluoresces brilliant green, peaking between 500 and 540 nm in most glass hosts. The brilliance of uranium fluorescence far exceeds that of the most efficient organic dyes. The fluorescence emission spectrum of hexavalent uranium-doped silica gel-glass is presented in Fig. 7. The peak maximum is at 533 nm [35]. 

In the excitation spectrum of europium (Fig. 6), there is a moderately strong peak at 532 nm, which is also where the fluorescence maximum of uranium exists (Fig. 7). This overlap provides a convenient opportunity for energy transfer between uranium and europium. Codoped silica gel was prepared with a Eu/U ratio of 20 1 [35]. The excitation spectrum for the 615 nm europium emission is shown in Fig. 8 for the codoped samples. For samples with the same europium content, the fluorescence emission is nearly doubled when sensitized by uranium at ideal pumping wavelengths [35]. At nonideal pumping... 

Figure 13 Fluorescence emission spectrum of europium-doped silica-PMMA composite.

A. Okubo (Japan) Anal Sci 2004 20 209 Multimode fiber core 800 pm, clad 1,000 pm material silica Xenon arc X = UV spectrum lOx objective Fluorescence... 

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