Supplementary Methods: Determination of the spectral

Transcription

1 Supplementary Methods: Determination of the spectral responsivity of fluorescence instruments. The spectral responsivity s( em ) of a fluorescence instrument like a fluorescence spectrometer can be best determined with a reference light source or so-called source-based standard 1-5. As shown in Box 1S, s( em ) is obtained from the quotient of the emission spectrum of the standard measured with the instrument to be calibrated under routine measurement conditions and the known emission spectrum of this standard 3-6. For the relative determination of fluorescence quantum yields, generally, a relative instrument characterization, focusing on the determination of the wavelength dependence of the relative shape of s( em ), is sufficient to consider wavelength- and polarization-dependent instrument-specific contributions to measured emission spectra 4, 7. Suitable reference light sources that are also termed spectral radiance transfer standards as they are used to transfer the value of one reference standard (here its wavelength-dependent spectral radiance as this is commonly used for a radiometric spectrometer characterization) to a measurement or to another reference standard 7, can be either physical devices like lamps (e.g. tungsten strip lamps or halogen lamps placed inside an integrating sphere, termed also integrating sphere radiators) or chemical reference materials, i.e., chromophore-based standards like solutions of organic dyes and glasses doped with transition and rare earth metal ions 2, 6, 8. In the latter case, the instrument-independent, i.e., corrected emission spectra of these chromophore-based standards must have been determined on a calibrated fluorometer with a given measurement uncertainty, thereby establishing typically traceability to the spectral radiance scale.

2 Requirements on physical and chemical standards for the determination of the spectral responsivity are broad and preferably unstructured spectra in the UV/vis/NIR spectral region that must be exactly known or even certified with a given uncertainty 3, 8, 9. Moreover, their emission spectra must be measurable with application-relevant instrument settings (e.g., emission slit width, detector voltage etc.) within the linear range of the detection system to enable the determination of s( em ) at exactly the same instrument settings (except for the excitation slit width) as used for the measurement of emission spectra of samples and quantum yield standards 8. This can be best met with reference light sources, the spectral radiances (or emitted light intensities) of which are in the same order of magnitude as those of typically measured samples and quantum yield standards 6, 10. Otherwise, the spectral radiance of the reference light source has to be attenuated without the introduction of additional spectral contributions which requires sophisticated attenuation procedures 3. Suitable procedures for the determination of s( em ) with a source-based standard like a calibrated lamp and with chromophore-based standards are exemplary shown in Box 1S, Figure 1S and Figure 2S and detailed in the PROTOCOL section. Prerequisites for these procedure are i.) control of the wavelength accuracy of the instrument s detection channel e.g., with a wavelength standard like an atomic discharge lamp and ii.) knowledge of the linear range of the detection system 2, 7. These procedures can yield the relative shape of s( em ) with an uncertainty of 5 10 %, using a confidence interval of 95 % 7. In a recent study of four National Metrology Institutes (NMIs) on the state-of-the art comparability of corrected emission spectra of several test dyes using different calibration procedures and physical transfer standards like calibrated lamps and white standards, a comparability of corrected emission spectra within a relative standard deviation of 4.2 % was achieved 1.

3 Determination of the spectral responsivity s( em ) with a reference light source - Prerequisite for the determination of the spectral responsivity s( em ) of a fluorescence instrument is a light source, the emission spectrum or wavelength-dependent spectral radiance of which is precisely known (see Figure a). - The quotient of the signal measured with the instrument to be calibrated at routinely used measurement conditions (Figure b) and the known emission spectrum of the reference light source yields the (relative) spectral responsivity s( em ) or its reciprocal, the emission correction curve 1/ s( em ) (see Figure c)). - Multiplication of measured (spectrally) uncorrected emission spectra (Figure d) with the emission correction curve 1/ s( em ) yields corrected instrument-independent emission spectra I c (Figure e), which can be then used for the calculation of fluorescence quantum yields (see Boxes 1 and 2 in the manuscript).

4 Box S1: Determination of the spectral responsivity s( em ) with a reference light source.

5 PROCEDURES Procedure A: Determination of the spectral responsivity of fluorescence instruments with a reference light source 1) Switch the reference light source on and allow it to thermally equilibrate (wait for the time recommended by the standard s manufacturer for thermal equlibration before recording the standard s emission spectrum; if no information are provided, as a rule of thumb, a thermal equilibration time of two hours should be sufficient). Use only the power supply that was employed for the calibration of the reference light source. 2) Positioning of the reference light source. (i) In the case of a fluorescence spectrometer, the reference light source (commonly calibrated wavelength dependence of the spectral radiance, see Box 1S) can be placed inside the sample compartment at sample position only if its spectral radiance is not too high to cause detector saturation or detector operation in a nonlinear range 2, 7. For intense reference light sources like tungsten strip lamps or integrating sphere radiators, the spectral radiances of which exceed those of common fluorescent samples by ca. two to four orders of magnitude, the standard needs to be attenuated 3, 10. This attenuation can render the positioning of the lamp at a different place necessary. A suitable attenuation procedure assessed and employed by us presents the use of a white standard or diffuse reflector that is placed at sample position and illuminated

6 by the calibrated source-based standard 1, 3, 7. In this case, the spectral radiance factor of the white standard must be known for the measurement geometry used for instrument calibration, i.e., for the actual angles between incident light, reflected light, and reflector surface. This procedure enables simple adjustment of the light intensity of the source-based standard reaching the instrument s detector by variation of the distance between the reference light source and the reflector. (ii) In the case of an integrating sphere setup, the reference light source is placed in front of the excitation port of the integrating sphere 11, 12. In this case, the light intensity (or spectral radiant power) reaching the detector can be adjusted by a variation of the distance between the standard and the integrating sphere entrance port 11. 3) Record the emission spectrum of reference light source for each measurement condition used for the luminescence measurements to be subsequently corrected. This includes identical instrument settings like emission slit width, detector voltage etc., the same optical components in the detection channel like filters, diffraction gratings, same settings of the emission polarizer, same wavelength grid in the case of charge coupled device (CCD) detectors. 4) Make sure that the detector is always operated within its linear range. Recommendations of how the linear range of a detector can be simply and reliably determined are given in references 1, 3, 7. Care has to be taken if the procedure described in ASTM E is

7 used as the recommended dye concentrations are too high, favoring inner filter effects. Moreover, fluorescence intensity is proportional to absorption factor and not to absorbance 13. 5) Calculate the spectral responsivity s( em ) for each measurement condition by dividing the recorded emission spectrum of the source-based standard by the known spectrum of the reference light source as shown in Box 1S 1-3. Although this procedure is challenging and typically only advisable for expert laboratories 2, 7, the use of calibrated lamps presents currently the only option to determine s( em ) for wavelength below 300 nm and above ca. 770 nm due to the lack of suitable and reliable chromophore-based standards for the UV and the NIR. For example, the Kit Spectral Fluorescence Standards covers only the emission wavelength region from 300 nm to 770 nm 1, 3, 14, 15. Procedure B: Determination of the spectral responsivity of fluorescence instruments with chromophore-based standards As an alternative to physical standards, chromophore-based standards can be employed for the determination of s( em ) otherwise using the same instrument characterization procedure as detailed in Box 1S for a standard lamp 1, 2, 7. This is recommended for the majority of fluorescence users 7, 15. The use of chromophore-based standards elegantly circumvents the need for sophisticated attenuation procedures 3, 14. In addition, it is more cost efficient as it does not rely on expensive calibrated physical devices that must be regularly recalibrated.

8 A drawback of chromophore-based standards presents their - compared to a standard lamp - narrow emission spectra that cover a spectral window of at maximum ca. 150 nm per chromophore in the UV/vis region 3. The determination of s( em ) with chromophore-based standards in the UV/vis/NIR region thus requires the use of a set of several emission standards with crossing emission spectra 9, 14. Such a set, the Kit Spectral Fluorescence Standards F001 to F005 that consists of five strongly fluorescent organic dyes with broad emission bands and a nearly isotropic fluorescence used as ethanolic solutions, was developed by us for the wavelength region from 300 nm to 770 nm 6, 14 and is commercially available (Sigma Aldrich, KT-F). F001 to F005 are commercialized together with an evaluated standard operation procedure for their use, see also Figure 2S, and a custom-made data evaluation software LINKCORR for the calculation of the overall emission correction curve 1/s( em ) from the individual spectral responsivities or individual emission correction curves obtained for each dye (within the spectral region covered by its emission band) in a similar fashion as described for the use of a standard lamp in Box 1S 14. This is shown in Figure 1S. This procedure is representative for the determination of s( em ) with chromophore-based standards 3, 6, 7, 9, 13, A flow chart describing the determination of the spectral responsivity with these standards is summarized in Figure S2. The corrected emission spectra of F001 to F005 as well as the previously reported procedure used for the determination of s( em ) (or 1/s( em )) with these spectral fluorescence standards and LINKCORR 3, 14 were only recently evaluated in an international laboratory comparison of four National Metrology Institutes on the state-of-the art comparability of corrected emission spectra 1. A recent assessment of the calibration performance of twelve field laboratories using F001 to

9 F005 and LINKCORR demonstrated a comparability of the corrected emission spectra of three test dyes within a relative standard deviation of 6.8 % 15. Figure S1: Determination of the emission correction curve 1/s( em ) with spectral fluorescence standards like F001 to F005. Bottom: Measured uncorrected emission spectra (dotted lines) and BAM-certified corrected emission spectra (solid lines); Top: Wavelength-dependent quotients of the measured uncorrected and the certified corrected emission spectra (symbols) obtained for each spectral fluorescence standard within the spectral region covered by its emission spectrum and the resulting overall emission correction curve 1/s( em ) obtained by merging of these quotients. In the case of the Kit Spectral Fluorescence Standards from BAM, the evaluated software LINKCORR that contains the certified corrected emission spectra of F001 to F005, calculates these quotients and merges them following a slightly modified procedure as reported by Gardecki 9.

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11 Figure 2S: Flowchart for use of F001 to F005, dyes and solvents are available from Sigma Aldrich and BAM. Other commercialized spectral fluorescence standards are fluorescent glasses from the National Institute for Standards and Technology (NIST), 2, 6 i.e., four cuvette-shaped glasses Standard Reference Materials (SRM) SRM , SRM , SRM , and SRM that cover the wavelength region of 320 nm to 780 nm as a set. Disadvantages of these glasses as compared to the liquid spectral fluorescence standards F001 to F005 are their typically low fluorescence quantum yields (below 0.1), their long fluorescence lifetimes in the s to ms region that can hamper their use for instruments equipped with pulsed light sources such as many routine fluorometers (the shape of emission spectra measured on instruments with pulsed excitation and gated detection differ from the certified glass spectrum e.g. for SRM 2940) 17, the slightly structured emission spectrum of SRM , and their geometric restrictions. Moreover, the determination of an overall emission correction curve for the wavelength region of 320 nm to 720 nm with these four glasses requires matching of the individual emission correction curves obtained for each glass 9 which can result in enhanced uncertainties as compared to the use of F001 to F005 where this matching is performed automatically by LINKCORR 3. Also, there is no blank provided with these glasses. Reference quantity of emission correction curves Knowledge of the reference quantity of 1/s( em ) is of considerable importance for the determination of fluorescence quantum yields as this reference quantity determines whether the photonic nature of the emitted photons has to be subsequently considered prior to integration of the blank and spectrally corrected emission spectra of the sample and the standard by

12 multiplication with the term em /(hc 0 ); reference quantity spectral radiance L ( )) or not (reference quantity spectral photon radiance L p, ( ); L p, ( ) = /(hc 0 ) L ( ); multiplication with em /(hc 0 ) to be omitted) 3, 20. Determination of s( em ) with reference light sources like calibrated lamps establishes traceability of the emission correction curve area as commonly, the wavelength dependence of the spectral radiance of these lamps is calibrated or certified, and not their spectral photon radiances. The corrected emission spectra of the chromophore-based standards F001 to F005 and the fluorescent glasses or quinine sulfate dihydrate from NIST are also traceable to the spectral radiance as the calibration of the fluorescence spectrometer at BAM and NIST used for the determination of their corrected emission spectra were calibrated with calibrated lamps of known spectral radiance 14, 16-19, 21. This is always explicitly stated in the corresponding certificates from BAM and NIST. Critical can be the use of emission correction curves implemented by instrument manufacturers as many manufacturers do not provide sufficient information on the calibration procedures and standards used (and on the instrument settings employed). Until now, no internationally accepted agreement on the reference quantities to be used for spectrometer characterization was reached, only recommendations by the International Union of Pure and Applied Chemistry (IUPAC; project # ) 7 and ASTM International (formerly American Society for Testing and Materials) 2 regarding the use of the spectral radiance as reference quantity for emission correction curves. However, as a group of fluorescence experts suggested the use of photonic units for corrected emission spectra, which equals the traceability of these spectra to the spectral photon radiance 22, some instrument manufacturers like Edinburgh Instruments use the spectral photon radiance as reference quantity for their implemented emission correction curves. In this case, the term em in Box 1 (step B) and Box 2 must be omitted.

13 If the reference quantity of instrument-implemented correction curves is not clear, it is strongly recommended to contact either the instrument manufacturer to clarify the reference quantity used. Otherwise, the emission correction curves need to measured as previously described in Box 1S and Figures 1S and 2S. Reabsorption correction for integrating sphere measurements. Reabsorption effects are indicated by a red shift of the emission maximum with increasing dye concentration and thus, absorption (see Box 4: Hints for troubleshoot and Table 2 in the manuscript). These effects can be mathematically corrected if an undisturbed, spectrally corrected emission spectrum I c (λ em ) of the fluorophore studied is available. Such a spectrum can be measured with the integrating sphere setup using a very dilute dye solution and several scans or more straightforward with acalibrated fluorescence spectrometer (A = 0.1 at the longest wavelength absorption maximum). The latter measurements are less prone to reabsorption effects as compared to measurements with integrating sphere setups, thus enabling the use of higher dye concentrations and accordingly shorter measurement times. In a spectral region where no overlap occurs between absorption and emission, the spectrally corrected, (reabsorption) distorted fluorescence spectrum I c,d (λ em ) measured with the integrating sphere setup is matched with the undisturbed emission spectrum I c (λ em ) (see Box: Hints for troubleshoot in the manuscript, Reabsorption). Comparison of the integrated emission intensities yields a factor w which can be interpreted as the reabsorption probability (see equation S1) em em I ( ) d c,d em em I ( ) d c em em 1 w (eq. S1)

14 The observed quantum yield (see Box 2 in the manuscript), termed here f,obs, can be corrected for reabsorption effects with equation S2, which subsequently leads to reabsorption-corrected quantum yields 11, 12, 23 f,cor. f,cor f,obs 1 w w f,obs (eq. S2) 1. Resch-Genger, U. et al. State-of-the Art Comparability of Corrected Emission Spectra.1. Spectral Correction with Physical Transfer Standards and Spectral Fluorescence Standards by Expert Laboratories. Anal. Chem. 84, (2012). 2. DeRose, P.C. & Resch-Genger, U. Recommendations for Fluorescence Instrument Qualification: The New ASTM Standard Guide. Anal. Chem. 82, (2010). 3. Resch-Genger, U. et al. Traceability in fluorometry: Part II. Spectral fluorescence standards. J. Fluoresc. 15, (2005). 4. Hollandt, J. et al. Traceability in fluorometry - Part I: Physical standards. J. Fluoresc. 15, (2005). 5. Resch-Genger, U. & derose, P.C. Characterization of Photoluminescence Measuring Systems (IUPAC Technical Report). Pure Appl. Chem. 84, (2012). 6. Resch-Genger, U., Hoffmann, K. & Pfeifer, D. Simple Instrument Calibration and Validation Standards for Fluorescence Techniques. in Reviews in Fluorescence (ed. C.D. Geddes) 1-32 (Springer Science Businesss Media, Inc., New York, 2009). 7. Resch-Genger, U. & derose, P.C. Characterization of Photoluminescence Measuring Systems (IUPAC Technical Report). Pure Appl. Chem. 84, (2012). 8. Resch-Genger, U. & derose, P. Fluorescence standards: Classification, terminology, and recommendations on their selection, use, and production (IUPAC Technical Report). Pure Appl. Chem. 82, (2010). 9. Gardecki, J.A. & Maroncelli, M. Set of Secondary Emission Standards for Calibration of the Spectral Responsivity in Emission Spectroscopy. Appl. Spectrosc. 52, (1998). 10. Monte, C., Resch-Genger, U., Pfeifer, D., Taubert, D.R. & Hollandt, J. Linking fluorescence measurements to radiometric units. Metrologia 43, S89-S93 (2006). 11. Würth, C., Pauli, J., Lochmann, C., Spieles, M. & Resch-Genger, U. Integrating Sphere Setup for the Traceable Measurement of Absolute Photoluminescence Quantum Yields in the Near Infrared. Anal. Chem. 84, (2012). 12. Würth, C., Grabolle, M., Pauli, J., Spieles, M. & Resch-Genger, U. Comparison of Methods and Achievable Uncertainties for the Relative and Absolute Measurement of Photoluminescence Quantum Yields. Anal. Chem. 83, (2011). 13. Lakowicz, J.R. Principles of fluorescence spectroscopy (ed. J.R. Lakowicz) (Springer Science+Business Media, LLC, New York, 2006).

15 14. Pfeifer, D., Hoffmann, K., Hoffmann, A., Monte, C. & Resch-Genger, U. The calibration kit spectral fluorescence standards - A simple and certified tool for the standardization of the spectral characteristics of fluorescence instruments. J. Fluoresc. 16, (2006). 15. Resch-Genger, U. et al. State-of-the Art Comparability of Corrected Emission Spectra. 2. Field Laboratory Assessment of Calibration Performance Using Spectral Fluorescence Standards. Anal. Chem. 84, (2012). 16. DeRose, P.C., Smith, M.V., Mielenz, K.D., Blackburn, D.H. & Kramer, G.W. Characterization of standard reference material 2941, uranyl-ion-doped glass, spectral correction standard for fluorescence. J. Lumin. 128, (2008). 17. DeRose, P.C., Smith, M.V., Mielenz, K.D., Blackburn, D.H. & Kramer, G.W. Characterization of Standard Reference Material 2940, Mn-ion-doped glass, spectral correction standard for fluorescence. J. Lumin. 129, (2009). 18. DeRose, P.C., Smith, M.V., Mielenz, K.D., Anderson, J.R. & Kramer, G.W. Characterization of Standard Reference Material 2943, Cu-ion-doped glass, spectral correction standard for blue fluorescence. J. Lumin. 131, (2011). 19. DeRose, P.C., Smith, M.V., Mielenz, K.D., Anderson, J.R. & Kramer, G.W. Characterization of Standard Reference Material 2942, Ce-ion-doped glass, spectral correction standard for UV fluorescence. J. Lumin. 131, (2011). 20. Braslavsky, S.E. Glossary of terms used in Photochemistry 3(rd) Edition (IUPAC Recommendations 2006). Pure Appl. Chem. 79, (2007). 21. Velapoldi, R.A. & Mielenz, K.D. A Fluorescence Standard Reference Material: Quinine Sulfate Dihydrate. NBS Special Publication , (1980). 22. Chapman, J.H. et al. Proposal for the standardization of reporting fluorescence emission spectra. Appl. Spectrosc. 17, (1963). 23. Würth, C. et al. Evaluation of a Commercial Integrating Sphere Setup for the Determination of Absolute Photoluminescence Quantum Yields of Dilute Dye Solutions. Appl. Spectrosc. 64, (2010).