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A model of heavy atom-analyte-substrate for room-temperature phosphorescence inducement on filter papers
MICROCHEMICAL
JOURNAL
36, 118-121 (1987)
A Model of Heavy Atom-Analyte-Substrate for Room-Temperature Phosphorescence Inducement on Filter Papers’ S. Y. Su AND J. D. WINEFORDNER*,* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284 and *Department of Chemistry, Universiry of Florida, Gainesville, Florida 32611 Received January 19, 1987; accepted March 24, 1987 A model of the heavy atom-analyte-substrate interaction in room-temperature phosphorimetry is proposed. The model is based on previous experimental results involving x-ray photoelectron spectroscopy in one of the authors’ laboratories. o 1987 Academic press, Inc.
ACTIVE SITES IN CELLULOSE SUBSTRATE
Three classes of carbons are present in the cellulose substrate as discussed previously (I): C-I, C-II, and C-III. Class I carbons (C-I) are those bonded to only carbon or hydrogen atoms; class II and III carbons (C-II and C-III) are those bonded to one and two oxygens, respectively. According to the molecular structure of glucose, the unit of cellulose polymer as shown in Fig. 1, there are no C-I carbons but there are five C-II carbons and one C-III carbon in each molecule. The detection of C-I carbons in cellulose substrates previously has been reported to come from the residual impurities (I). Among the C-II carbons, there are two groups of carbon present. One is the carbon which is bonded to a hydroxyl group, -C-(OH), and the other to an oxygen and a carbon, -C-O-C, which is designated as the C-II-a carbon in Fig. 1. Furthermore, there are two types of C-(OH) arrangements, namely primary and secondary alcohols, i.e., -C-CH,-C-(H)(OH), respectively, which are designated in Fig. 1 as C-II-b and C-II-c carbons, respectively. These three types of carbon-oxygen arrangements of C-II carbons may cause differences in the degree or strength of hydrogen-bond formation, ionic attraction/repulsion, and other interactions. These interactions were reported to account for the inducement of room-temperature phosphorescence (RTP) of some types of compounds (2). Nonetheless, there was insuflkient information in the previous paper (I) to evaluate individual contributions of these three types of carbons to the RTP signal. THE ROLE OF HEAVY ATOMS
Based on previous studies of optimal heavy atom concentrations for RTP observation (3), the optimal heavy atom and concentration is I- at 1 M. A l-cm r Research supported by NIH-GM 11373-24. * To whom all correspondence should be addressed. 118 0026-265X/87 $1.50 Copyright 0 1987 by Academic Press, Inc. All r@hts of reproduction in any fom reserved.
MODEL
OF HEAVY ATOM-ANALYTE-SUBSTRATE
119
Whatman No. 1 paper disk weighs approximately 6 mg, which contains about 2 x 1019 glucopyranosyl molecules (C6Hi005). When a 6-p,l 1.0 M KI solution was applied to the substrate, 3.6 x 1018 atoms of I- have been applied. Each glucopyranosyl molecule contains six carbon atoms. Therefore, the elemental ratio of I/C should be 0.03. If the 8 to 10% impurities of C-I carbons are considered, however, the elemental ratio of I/(C-II + C-III) should be 0.033. The reported x-ray photoelectron spectroscopy (XPS) experimental value (I) of I/(C-II + C-III) was 0.014. Possible contributions to this discrepancy include the irregularity of cellulose substrate, the n-reproducibility of the fractional area measured and calculated by XPS, the nonideality of the luminescence probe, and the loss of I- during sample application. Another possible reason for this is that cellulose is a high molecular weight polymer which may restrict the interaction between Iand cellulose substrate among glucopyranosyl molecules on the surface of the polymer; i.e., inner glucopyranosyl molecules will not interact with I-. According to the previous report (I), heavy atoms and organic molecules were distributed uniformly on the substrate; however, the extent of penetration of the heavy atoms or ions was observed to exceed that of the organic molecules. This observation is true only within the depth examined by the XPS technique, which lies between 15 and 100 A. To know the exact I/C ratio used in RTP inducement, the depth and homogeneity of penetration of the excitation radiation into the cellulose substrate should be known. It is recognized that pretilter and postfilter effects are the two major factors which account for the nonlinearity of luminescence measured at higher concentrations of heavy atom for a given analyte. There should be a certain depth of penetration for the excitation radiation to cause the pre- and postfilter effects in cellulose substrates. In addition, the amount of a heavy atom added and thus the concentration of heavy atom within the solid substrate causes the nonlinearity of RTP, which is evidenced by previous heavy atom studies (3). Iodide, for example, usually induces nonlinearity above 1 M concentration. Therefore, the ratio of molar amounts of heavy atom to molar amounts of substrate should have an optimal value for RTP inducement. This leads to the development of the following model for the heavy atom-substrate system. According to the above discussion, the I/(C-II + C-III) ratio lie between 0.014 and 0.033, which gives an Uglucopyranosyl molecule ratio of 0.08 to 0.2. This means that an iodide ion is shared by several glucopyranosyl molecules. Therefore, the electron cloud density of the anionic heavy atom, such as I-, is believed to repel the electron clouds surrounding oxygen atoms which are bonded to C-II
H
FIG. 1, Structure of glucopyranosyl molecule as a basic unit in cellulose substrate.
SU AND WINEFORDNER
120
and C-III carbons. Such repulsion results in “pushing” the electron distribution closer to the carbon atoms bonded to the oxygen atoms. Such an electron cloud shift toward carbon atoms would strengthen the bonding between carbon and oxygen atoms. This explains two previous (1) observations: first, that I- did not interact strongly with phosphors due to its interaction with oxygen atoms; and second, that bond energies of C-II and C-III carbons increased when I- was applied on cellulose substrates, due to the increase of the electron cloud distribution between oxygen and carbon atoms. As listed in Table 1, photoelectron binding energies of C-II and C-III carbons are 0.9 and 1.5 eV, respectively. If the binding energies reported for C-II and C-III carbons are average values, the total binding energy change for C-II carbons should be five times that of 0.9 or 4.5 eV, and that for C-III carbons should be 1.5 eV. This implies that the two oxygen atoms bonded to C-III experience an increased electron cloud density compared with that of the oxygen atoms bonded to C-II carbons. Internal heavy atoms, such as iodide in 3,%diiodotyrosine, will have a similar effect of electron cloud repulsion of external heavy atoms. Cationic heavy atoms, such as Tl+, Ag+, and Pb+*, are also used for RTP inducement of nonionizable polycyclic aromatic hydrocarbons (PAHs); therefore, they should act differently from anionic heavy atoms. The model for cationic heavy atoms is not proposed here due to the lack of experimental data. THE ROLE OF ANALYTES
The photoelectron bonding energies of C-II and C-III carbons decreased when the electron cloud density surrounding the oxygen atoms decreased. The decrease in electron density of an oxygen atom, which subsequently weakens the bonding between carbon and oxygen atoms, will occur when a hydrogen bond is formed between an analyte and the oxygen atom. The decreases in bonding energies of C-II and C-III carbons were reported to be 0.6 eV for C-II and 0.7 eV for C-III carbons when 0.01 M 5-diiodotyrosine was applied on a 0.1 M I--treated XPS Photoelectron
TABLE 1 Bonding Energy and Chemical Shifts for Carbons and Oxygen
Spotting solutiona
C-II”
AC-IIb
C-III”
AC-IIIb
Ob
Aob
Blank (substrate) l.OMKI 1 .O M KI and 0.01 M 5hydroxytryptophan 2 x 10-3 M 3,S-diiodotyrosine 0.1 M Tl NO, 0.1 M TlNO, and 5 x lo-) M carbaryl
286.4 287.3
0.9
287.6 289.1
1.5
532.8 533.9
0.9
286.7
0.3
288.4
0.8
533.4
0.6
286.7 287.3
0.3 0.9
288.4 284.0
0.8 -3.6
533.7 534.0
0.9 1.2
287.2
0.8
289.0
1.4
534.0
1.2
a Data were taken from Ref. (I). All values are in eV. b Chemical shifts were obtained by subtracting bonding energies of respective carbons and oxygen from those of the blank.
MODEL
OF HEAVY
ATOM-ANALYTE-SUBSTRATE
121
cellulose substrate. The decrease in bonding energies did not arise from the interaction between the analyte and the heavy atom, according to the previous observation (I). Therefore, the decreases are believed to arise from the interaction between the analyte and the oxygen atoms through hydrogen bonding or ionic interaction. However, almost no change in bonding energies of C-II and C-III carbons was observed when 0.1 M TlNO, was used as the heavy atom and 5 x 10e3 M 3.5 diiodotyrosine was applied. A similar observation was reported for 0.1 M TlNO,/ 5 x 1O-3 M carbaryl; this observation concurs with the observation that carbaryl gives no RTP or very weak RTP unless I- is used as the heavy atom. The decreases in oxygen bonding energies when analytes were applied also support the above model. The formation of hydrogen bonds or ionic interactions between analytes and glucopyranosyl molecules involved electrons surrounding oxygen atoms in the -C-Obonds. This would result in a decrease of electron density of the oxygen bond as well as of its bonding energy. In conclusion, the previous XPS study (I) supplied bonding energy information which is used in this report to support the model proposed for the phosphorescence inducement of ionizable analyte with anionic heavy atoms. This model of anionic heavy atom-ionizable analyte-cellulose substrate satisfies most theories developed previously for the RTP technique. In addition, the lack of effect of the RTP signal on the order of application of heavy atom and analyte solutions can also be justified because the heavy atom, I-, does not interact with the analyte but does with glucopyranosyl substrate whereas the analyte also interacts with the cellulose but does not with the heavy atom. However, more work needs to be done to further improve upon this model in order to know the relative position of heavy atom and analyte which will enable greater intersystem crossing for phosphorescence inducement. Another model for cationic heavy atom-PAH system also is needed. ACKNOWLEDGMENTS The authors thank Sheila Rosentield at the Reynolds Metals Co. for her comments on the photoenergy shifts and Dr. E. B. Asafu-Adjaye for his comments on the manuscript.
REFERENCES 1. Andino, M. M.; Kosinski, M. A.; Winefordner, J. D. Anal. Chem., 1986, 58, 1730. 2. Vo-Dinh, T. In Chemical Analysis (P. J. Elving and J. D. Winefordner, Eds.), Vol. 68, Chap. 2, references cited within. 3. Su, S. Y.; Winefordner, J. D. Canad. J. Spectrosc., 1983, 28, 21.
JOURNAL
36, 118-121 (1987)
A Model of Heavy Atom-Analyte-Substrate for Room-Temperature Phosphorescence Inducement on Filter Papers’ S. Y. Su AND J. D. WINEFORDNER*,* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284 and *Department of Chemistry, Universiry of Florida, Gainesville, Florida 32611 Received January 19, 1987; accepted March 24, 1987 A model of the heavy atom-analyte-substrate interaction in room-temperature phosphorimetry is proposed. The model is based on previous experimental results involving x-ray photoelectron spectroscopy in one of the authors’ laboratories. o 1987 Academic press, Inc.
ACTIVE SITES IN CELLULOSE SUBSTRATE
Three classes of carbons are present in the cellulose substrate as discussed previously (I): C-I, C-II, and C-III. Class I carbons (C-I) are those bonded to only carbon or hydrogen atoms; class II and III carbons (C-II and C-III) are those bonded to one and two oxygens, respectively. According to the molecular structure of glucose, the unit of cellulose polymer as shown in Fig. 1, there are no C-I carbons but there are five C-II carbons and one C-III carbon in each molecule. The detection of C-I carbons in cellulose substrates previously has been reported to come from the residual impurities (I). Among the C-II carbons, there are two groups of carbon present. One is the carbon which is bonded to a hydroxyl group, -C-(OH), and the other to an oxygen and a carbon, -C-O-C, which is designated as the C-II-a carbon in Fig. 1. Furthermore, there are two types of C-(OH) arrangements, namely primary and secondary alcohols, i.e., -C-CH,-C-(H)(OH), respectively, which are designated in Fig. 1 as C-II-b and C-II-c carbons, respectively. These three types of carbon-oxygen arrangements of C-II carbons may cause differences in the degree or strength of hydrogen-bond formation, ionic attraction/repulsion, and other interactions. These interactions were reported to account for the inducement of room-temperature phosphorescence (RTP) of some types of compounds (2). Nonetheless, there was insuflkient information in the previous paper (I) to evaluate individual contributions of these three types of carbons to the RTP signal. THE ROLE OF HEAVY ATOMS
Based on previous studies of optimal heavy atom concentrations for RTP observation (3), the optimal heavy atom and concentration is I- at 1 M. A l-cm r Research supported by NIH-GM 11373-24. * To whom all correspondence should be addressed. 118 0026-265X/87 $1.50 Copyright 0 1987 by Academic Press, Inc. All r@hts of reproduction in any fom reserved.
MODEL
OF HEAVY ATOM-ANALYTE-SUBSTRATE
119
Whatman No. 1 paper disk weighs approximately 6 mg, which contains about 2 x 1019 glucopyranosyl molecules (C6Hi005). When a 6-p,l 1.0 M KI solution was applied to the substrate, 3.6 x 1018 atoms of I- have been applied. Each glucopyranosyl molecule contains six carbon atoms. Therefore, the elemental ratio of I/C should be 0.03. If the 8 to 10% impurities of C-I carbons are considered, however, the elemental ratio of I/(C-II + C-III) should be 0.033. The reported x-ray photoelectron spectroscopy (XPS) experimental value (I) of I/(C-II + C-III) was 0.014. Possible contributions to this discrepancy include the irregularity of cellulose substrate, the n-reproducibility of the fractional area measured and calculated by XPS, the nonideality of the luminescence probe, and the loss of I- during sample application. Another possible reason for this is that cellulose is a high molecular weight polymer which may restrict the interaction between Iand cellulose substrate among glucopyranosyl molecules on the surface of the polymer; i.e., inner glucopyranosyl molecules will not interact with I-. According to the previous report (I), heavy atoms and organic molecules were distributed uniformly on the substrate; however, the extent of penetration of the heavy atoms or ions was observed to exceed that of the organic molecules. This observation is true only within the depth examined by the XPS technique, which lies between 15 and 100 A. To know the exact I/C ratio used in RTP inducement, the depth and homogeneity of penetration of the excitation radiation into the cellulose substrate should be known. It is recognized that pretilter and postfilter effects are the two major factors which account for the nonlinearity of luminescence measured at higher concentrations of heavy atom for a given analyte. There should be a certain depth of penetration for the excitation radiation to cause the pre- and postfilter effects in cellulose substrates. In addition, the amount of a heavy atom added and thus the concentration of heavy atom within the solid substrate causes the nonlinearity of RTP, which is evidenced by previous heavy atom studies (3). Iodide, for example, usually induces nonlinearity above 1 M concentration. Therefore, the ratio of molar amounts of heavy atom to molar amounts of substrate should have an optimal value for RTP inducement. This leads to the development of the following model for the heavy atom-substrate system. According to the above discussion, the I/(C-II + C-III) ratio lie between 0.014 and 0.033, which gives an Uglucopyranosyl molecule ratio of 0.08 to 0.2. This means that an iodide ion is shared by several glucopyranosyl molecules. Therefore, the electron cloud density of the anionic heavy atom, such as I-, is believed to repel the electron clouds surrounding oxygen atoms which are bonded to C-II
H
FIG. 1, Structure of glucopyranosyl molecule as a basic unit in cellulose substrate.
SU AND WINEFORDNER
120
and C-III carbons. Such repulsion results in “pushing” the electron distribution closer to the carbon atoms bonded to the oxygen atoms. Such an electron cloud shift toward carbon atoms would strengthen the bonding between carbon and oxygen atoms. This explains two previous (1) observations: first, that I- did not interact strongly with phosphors due to its interaction with oxygen atoms; and second, that bond energies of C-II and C-III carbons increased when I- was applied on cellulose substrates, due to the increase of the electron cloud distribution between oxygen and carbon atoms. As listed in Table 1, photoelectron binding energies of C-II and C-III carbons are 0.9 and 1.5 eV, respectively. If the binding energies reported for C-II and C-III carbons are average values, the total binding energy change for C-II carbons should be five times that of 0.9 or 4.5 eV, and that for C-III carbons should be 1.5 eV. This implies that the two oxygen atoms bonded to C-III experience an increased electron cloud density compared with that of the oxygen atoms bonded to C-II carbons. Internal heavy atoms, such as iodide in 3,%diiodotyrosine, will have a similar effect of electron cloud repulsion of external heavy atoms. Cationic heavy atoms, such as Tl+, Ag+, and Pb+*, are also used for RTP inducement of nonionizable polycyclic aromatic hydrocarbons (PAHs); therefore, they should act differently from anionic heavy atoms. The model for cationic heavy atoms is not proposed here due to the lack of experimental data. THE ROLE OF ANALYTES
The photoelectron bonding energies of C-II and C-III carbons decreased when the electron cloud density surrounding the oxygen atoms decreased. The decrease in electron density of an oxygen atom, which subsequently weakens the bonding between carbon and oxygen atoms, will occur when a hydrogen bond is formed between an analyte and the oxygen atom. The decreases in bonding energies of C-II and C-III carbons were reported to be 0.6 eV for C-II and 0.7 eV for C-III carbons when 0.01 M 5-diiodotyrosine was applied on a 0.1 M I--treated XPS Photoelectron
TABLE 1 Bonding Energy and Chemical Shifts for Carbons and Oxygen
Spotting solutiona
C-II”
AC-IIb
C-III”
AC-IIIb
Ob
Aob
Blank (substrate) l.OMKI 1 .O M KI and 0.01 M 5hydroxytryptophan 2 x 10-3 M 3,S-diiodotyrosine 0.1 M Tl NO, 0.1 M TlNO, and 5 x lo-) M carbaryl
286.4 287.3
0.9
287.6 289.1
1.5
532.8 533.9
0.9
286.7
0.3
288.4
0.8
533.4
0.6
286.7 287.3
0.3 0.9
288.4 284.0
0.8 -3.6
533.7 534.0
0.9 1.2
287.2
0.8
289.0
1.4
534.0
1.2
a Data were taken from Ref. (I). All values are in eV. b Chemical shifts were obtained by subtracting bonding energies of respective carbons and oxygen from those of the blank.
MODEL
OF HEAVY
ATOM-ANALYTE-SUBSTRATE
121
cellulose substrate. The decrease in bonding energies did not arise from the interaction between the analyte and the heavy atom, according to the previous observation (I). Therefore, the decreases are believed to arise from the interaction between the analyte and the oxygen atoms through hydrogen bonding or ionic interaction. However, almost no change in bonding energies of C-II and C-III carbons was observed when 0.1 M TlNO, was used as the heavy atom and 5 x 10e3 M 3.5 diiodotyrosine was applied. A similar observation was reported for 0.1 M TlNO,/ 5 x 1O-3 M carbaryl; this observation concurs with the observation that carbaryl gives no RTP or very weak RTP unless I- is used as the heavy atom. The decreases in oxygen bonding energies when analytes were applied also support the above model. The formation of hydrogen bonds or ionic interactions between analytes and glucopyranosyl molecules involved electrons surrounding oxygen atoms in the -C-Obonds. This would result in a decrease of electron density of the oxygen bond as well as of its bonding energy. In conclusion, the previous XPS study (I) supplied bonding energy information which is used in this report to support the model proposed for the phosphorescence inducement of ionizable analyte with anionic heavy atoms. This model of anionic heavy atom-ionizable analyte-cellulose substrate satisfies most theories developed previously for the RTP technique. In addition, the lack of effect of the RTP signal on the order of application of heavy atom and analyte solutions can also be justified because the heavy atom, I-, does not interact with the analyte but does with glucopyranosyl substrate whereas the analyte also interacts with the cellulose but does not with the heavy atom. However, more work needs to be done to further improve upon this model in order to know the relative position of heavy atom and analyte which will enable greater intersystem crossing for phosphorescence inducement. Another model for cationic heavy atom-PAH system also is needed. ACKNOWLEDGMENTS The authors thank Sheila Rosentield at the Reynolds Metals Co. for her comments on the photoenergy shifts and Dr. E. B. Asafu-Adjaye for his comments on the manuscript.
REFERENCES 1. Andino, M. M.; Kosinski, M. A.; Winefordner, J. D. Anal. Chem., 1986, 58, 1730. 2. Vo-Dinh, T. In Chemical Analysis (P. J. Elving and J. D. Winefordner, Eds.), Vol. 68, Chap. 2, references cited within. 3. Su, S. Y.; Winefordner, J. D. Canad. J. Spectrosc., 1983, 28, 21.