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Stokes Shift Spectroscopy Highlights Differences of Cancerous and Normal Human Tissues

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Stokes Shift Spectroscopy Highlights Differences of Cancerous and Normal Human Tissues 3360 OPTICS LETTERS / Vol. 37, No. 16 / August 15, 2012 Stokes shift spectroscopy highlights differences of cancerous and normal human tissues Yang Pu, Wuabo Wang, Yuanlong Yang, and R. R. Alfano* Institute for Ultrafast Spectroscopy and Lasers, and Department of Physics, City College of the City University of New York, Convent Avenue at 138th Street, New York, New York 10031, USA *Corresponding author: [email protected] Received May 15, 2012; accepted June 25, 2012; posted June 28, 2012 (Doc. ID 168649); published August 6, 2012 The Stokes shift spectroscopy (S3) offers a simpler and better way to recognize spectral fingerprints of fluorophores in complex mixtures. The efficiency of S3 for cancer detection in human tissue was investigated systematically. The alterations of Stokes shift spectra (S3) between cancerous and normal tissues are due to the changes of key fluor- ophores, e.g., tryptophan and collagen, and can be highlighted using optimized wavelength shift interval. To our knowledge, this is the first time to explicitly disclose how and why S3 is superior in comparison with other conventional spectroscopic techniques. © 2012 Optical Society of America OCIS codes: 170.0170, 170.6510, 300.0300, 300.6170. Fluorescence spectroscopy has been widely investigated These differences may reflect tissue fluorophores’ for diagnosing cancer since the study by Alfano et al. in change during the evolution of cancer. In order to under- the 1980s [1]. The difference between the emission and stand which components mainly contribute to these absorption peaks is known as the Stokes shift interval, changes, S3 of the main fluorophores in breast tissue, Δλss. A spectroscopic method was proposed to acquire e.g., tryptophan, collagen, NADH, and flavin, need to be the fluorescence signal by a fixed wavelength shift inter- measured. S3 val (Δλi) between the excitation λexc and emission λem, The of a mixture solution of tryptophan, NADH, and which is termed as Stokes shift spectroscopy (S3) by flavin with Δλi 40 nm is displayed as solid line in 3 Alfano and Yang [2]. Although this spectral approach was Fig. 2(a).TheS of collagen for same value of Δλi used in multiple fluorophore analysis in tissue, no other was superposed as a dashed line. Since collagen is not groups in the United States are applying it in cancer de- soluble in water, it is hard to obtain the S3 of the mixture tection. This is because the previous studies did not in- including collagen. The aqueous collagen suspension was vestigate the reason why S3 is superior over absorption, shaken evenly before the measurements. Compared with fluorescence, and excitation-emission matrix (EEM) Fig. 1, Fig. 2(a) shows that the main peak at ∼290 nm measurements. Therefore, few researchers are aware for the S3 of the breast tissues is from tryptophan. The of the excellence of S3 technique in cancer diagnostic secondary main peak at ∼340 nm corresponds to col- application in tissues. lagen, and the very tiny peak at ∼380 nm stands for This Letter will demonstrate the efficiency of S3 to re- NADH. No obvious peak of flavin was observed. There- 3 cognize spectral fingerprints of fluorophores in complex fore, the S profiles of breast tissues acquired with Δλi mixtures and its application to highlight the difference 40 nm are mainly contributed from tryptophan, collagen, between cancerous and normal tissues. We will also and NADH. show how to select an optimal wavelength shift interval To study the relative content changes of fluorophores, to obtain the best Stokes shift spectra (S3) for the pur- an analytical method, namely the nonnegative least pose of cancer detection, and discuss why S3 is superior square (NNLS) method, was applied to the S3 of the over other conventional spectroscopic techniques. As an example, the averaging S3 of 15 pairs of cancer- ous (solid line) and normal (dash line) breast tissues 0.024 were recorded by setting Δλi 40 nm in the synchro- 0.16 nized scan mode of a spectrometer (Perkin-Elmer LS 50), 0.020 385nm and shown in Fig. 1. The scan speed was 300 nm per 0.016 minute. Each spectral profile was normalized to a unit 0.12 value of 1 (i.e., the sum of squares of the intensity ele- ~294 nm 0.012 ments in each emission spectrum was set as 1). The sali- 0.08 0.008 S3 370 380 390 400 410 ent difference of between cancerous and normal wavelength (nm) breast tissues can be observed as two reverses of the ~340 nm 0.04 peak intensities at ∼I294 and ∼I340: (1) Ic >In at Intensity (arb. units) cancer normal ∼294 nm, while Ic <In at ∼340 nm, where Ic and In are the intensity of cancerous and normal tissues, respec- 0.00 tively; and (2) I294 >I340 for cancer, while I294 <I340 for 300 400 500 600 normal, where I294 and I340 are the intensity at 294 and wavelength (nm) 340 nm, respectively. A tiny peak can be seen at 385 nm 3 Fig. 1. (Color online) Average Stokes Shift spectra of cancer- for the cancerous tissue by enlarging the S profile in the ous (solid) and normal (dash) breast tissues acquired by the range of 365 nm to 420 nm, as shown in the insert of Fig. 1. selective Δλc 40 nm. 0146-9592/12/163360-03$15.00/0 © 2012 Optical Society of America August 15, 2012 / Vol. 37, No. 16 / OPTICS LETTERS 3361 (a) 1000 cancerous and normal breast tissues, shown in Fig. 1 to 1000 (b) 800 extract relative contents of the fluorophores, e.g., trypto- 800 3 × 10 phan, collagen, and NADH, using the measured S 600 600 20nm signal acquired with Δλi 40 nm shown in Fig. 2(a). 60nm 80nm 400 400 Figure 2(b) shows the scatter plot of the relative content 100nm 120nm 200 200 Intensity (arb. units) for tryptophan versus collagen of cancerous (square) and Intensity (arb. units) 140nm normal (circle) breast tissues. A separating line on the 0 0 300400 500 600 300 400 500 600 scatter plots was loaded by the Linear discriminant Wavelength (nm) Wavelength (nm) analysis (LDA) model for the diagnostic significance of Fig. 3. (Color online) S3 of mixed solution of tryptophan, tryptophan versus collagen. The sensitivity and speci- NADH, and flavin obtained by (a) Δλc 20 nm (solid), 60 nm ficity were calculated as 80% and 86.7%, respectively. (dash), and 80 nm (dot); and (b) 100 nm (solid), 120 nm (dash), The most salient feature of Fig. 2(b) is that all data points and 140 nm (dot). for the normal tissue locate in the upper-left side in com- parison with the data for the cancerous tissue, indicating 3 Δλ that the relative contribution of collagen to the S signal When i reaches 120 nm, the NADH signal almost takes in the normal tissue is higher that of the cancerous over the flavin signal. S3 Δλ tissues, while the relative contribution of tryptophan in To quantitatively study the changes of with i, the S3 normal tissue is lower than that in the cancerous tissue. FWHM and the peak intensities of profiles as a func- Δλ Reproducible results were observed for other kinds of tion of i for three fluorophores in the solution are human tissue studies, such as prostate and lung cancers. shown as Figs. 4(a) and 4(b), respectively. Figure 4 is Δλi Similarly, two inverse spectral properties of (1) Ic >In at very useful for researchers to choose an optimal . For signal processing, two most important properties de- ∼294 nm, while Ic <In at ∼340 nm for 100% and termine the quality of signals: resolution and magnitude. (2) I294 >I340 for cancer, while I294 <I340 for normal for ∼60%, were observed in S3 measurements for cancer- The values of FWHM stand for the inversing resolutions, ous and normal tissues. the larger FWHM, and the worse resolution. Figure 3(a) In order to explicitly understand the diagnostic signif- actually reflects that the resolutions for tryptophan – – – icance of these two inverse properties, the S3 of mixed (square solid), NADH (circle dash), and flavin (hexagon Δλ solution of tryptophan, NADH, and flavin with the con- dot) decrease monotonously with the increase of i, ∼0 4 ∕ 3 indicating that the smaller Δλi, the higher the resolution centration of . mg cm was measured with different 3 of the S signal. Figure 3(b) exhibits (1) when Δλi Δλi from 20 to 140 nm with a step increase of 20 nm. The 20 nm is chosen, the three fluorophores have approxi- spectra of this mixture solution for Δλi 20, 60, and Δλ 80 nm are displayed as solid, dashed, and dotted lines, mately the same peak intensities; (2) as i grows up, respectively, in Fig. 3(a). For the visual reason, the S3 the peak intensities of three flurophores ascend at first, Δλ 20 but descend at different critical values of Δλi.Thecurve obtained by i nm was magnified by 10 times. – Δλ 80 Figure 3(b) exhibits the S3 of the same mixture solution for tryptophan (square solid) falls at i nm, for Δλ 100 flavin (hexagon–dot) drops at Δλi 60 nm, and for for i , 120, and 140 nm displayed as solid, – Δλ 120 dashed, and dotted lines, respectively. The spectrum of NADH (circle dash) decreases at i nm. Δλ 40 One should recognize that S3 actually acquires the the mixture acquired by i nm was shown in S3 Fig. 2(a) previously. All curves are acquired under same signal of fluorescence. The profile of each biomole- cule is determined by its corresponding peak positions experimental condition except using different Δλi.
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