During the phenomenon of fluorescence, light is absorbed and re-emitted directly after the excitation.[43] Light always occurs in quantized energy portions, called photons. A Jablonski diagram can be used to describe a fluorescence process regarding the quantum mechanical principles, see Figure 2.6. The interaction with light temporarily changes the electronic state of the system. A fluorescent system can be illustrated as a system consisting of two electronic states, a singlet ground state S0 and singlet first state S1. Each electronic state consists of additional substates, the vibrational energy levels, that are illustrated as dotted lines in Figure 2.6.
The excitation to a higher electronic energy level requires the absorption of a photon, which corresponds in the amount of energy to the difference between the two states. However, the excitation can also lead into the vibrational substates. The relaxation into the ground state can lead to an emission of a fluorescent photon, which corresponds to a radiative transition. A non-radiative transition can occur when the electron relaxes into another excited state without the emission of a photon or the absorbed energy is converted into heat. These two possible transitions lead to an energetic shift between the absorption and emission spectrum, which is calledStokes shift. The absorption and emission spectra of Atto655 are shown in picture (b) in Figure 2.6.
Besides the absorption and emission spectra further characteristics to describe a fluorophore are the fluorescence lifetime and quantum yield. The lifetime refers to the average time that a fluorophore stays in the excited state before returning to the ground state. As already mentioned two electronic transitions from the singlet first to the singlet ground state can occur. The radiative and non-radiative transition are given by the transition rateskr
(a)
Figure 2.6:Illustration to highlight the fluorescence process. Picture (a) shows a sim-plified Jablonski diagram, which describes the excitation of a system through the absorption of quantized light. The energy can be released in form of a fluorescent photon. Picture (b) shows the Stokes shift resulting from radiative and non-radiative transitions.
andknr, respectively. Thus, the fluorescence lifetimeτcan be described as follows:[43]
τ= 1
kr+knr. (2.6)
The fluorescence quantum yield QY is described by the ratio of emitted photons and absorbed ones, following equation 2.7:[43]
QY= kr kr+knr
=krτ. (2.7)
Here, the number of absorbed photons is a combination of the two transition rates kr and knr. The quantum yield states the probability of a radiative process. The quantum yield also defines, in addition to the extinction coefficient, how bright the fluorescent system can appear.[44]
In the Jablonski diagram in Figure 2.6 several interactions are excluded to highlight the fluorescence phenomenon. However, one additional state is illustrated, because it often appears during fluorescence spectroscopy measurements. It belongs to the so called forbidden transitions, because
the system undergoes a spin conversion from the singlet first state S1to the triplet state T. The transition from one excited main state to another is called inter-system crossing (ICS). The associated transition ratekICS is several orders of magnitude lower than for the transition between the singlet states.
If the system is located in the triplet state, the emitter is dark. This process can often be observed as a blinking of a fluorophore.
Experimentally the fluorescence lifetime can be determined by the excita-tion of the fluorescent system using a laser pulse and detecting the emitted photons in a time-resolved manner. The starting point is defined by the excitation laser pulse. From that moment on, the time is recorded until the arrival of the emission photon at the detector system. Time correlated single photon counting (TCSPC) represents a suitable experimental setup to determine the lifetime and is explained in the following section.
Time correlated single photon counting
In addition to the absorption and emission spectra, time-resolved mea-surements yield further information about fluorescent systems. Using time-resolved spectroscopy the studied sample is excited by short laser pulses and the fluorescence intensity decay dependent on time can be determined. Here, the direct detection of the time-dependent decay is challenging.
Fluorescence processes typically last several nanoseconds, which conse-quently have to be recorded with a temporal accuracy of picoseconds.[45,46]
To record the whole decay curve and not only a single fluorescence lifetime a high quantity of emitted photons is required. The measurement of the intensity decay would be impossible by the detection of only one excitation cycle of one emitter. The development of time correlated single photon counting (TCSPC) overcomes these challenges. Using a periodic laser exci-tation data over multiple exciexci-tation and emission cycles can be collected, so that the profile of the intensity decay can be reconstructed.
Figure 2.7 illustrates the reconstruction of a decay profile by TCSPC. It is based on the precisely temporal detection of emitted single photons, which are referenced to the excitation laser pulse. The small green profile represents the excitation laser pulse, which generates the large fluorescence intensity decay, illustrated in red. The small red profiles represent the detected emitted photons, which are summed in a histogram. The photon counting histogram depicts a fluorescence decay after a high number of
ex-citation periods. The principle of TCSPC shown in Figure 2.7 is simplified to highlight the reconstruction of the fluorescence intensity profile. Normally, the TCSPC setup is adjusted, so that less than one photon is recorded after one excitation pulse on an average. Typical counts rates are approximately 1 photon per 100 excitation pulses.[43]Larger count rates lead to a bias of shorter fluorescence lifetimes.
Figure 2.7:Illustration to highlight the principle of time correlated single photon counting.
Excitation laser pulses (small green profiles) lead to single photons (small red profiles), which can be detected dependent on time. The histogram of all counted photons reconstructs the fluorescence intensity decay.
Sensitive detectors for single photon counting are the photomultiplier tube (PMT) and the single-photon avalanche photodiode (SPAD).[43]PMTs typically show low counting efficiencies between 10 % and 40 %, which is limited by the frequent generation of a photoelectron by the photoelectric effect of an incident photon. This efficiency is termed quantum efficiency.
Using SPADs photon counting with high efficiencies of over 70 % and low noise are realized.[47] However, a smaller active area can complicate the measurement.
Fluorescence correlation spectroscopy
Fluorescence correlation spectroscopy (FCS) was firstly introduced in 1972 byMagde,ElsonandWebb.[48]Using this method they were able to explore thermodynamic concentration fluctuations within the binding of ethidium bromide to DNA. Only two years later further detailed publications were published, where the theoretical background and experimental applicability of FCS was outlined.[49,50]Since then, biologists, biophysicists, chemists and physicists applied this experimental technique to investigate fluctuations of fluorescence in dynamic molecular systems.[51–53] FCS exhibits several advantageous properties, such as:[53]
• Small amounts and low concentrations of sample are required,
• investigation ofin situmeasurements is possible,
• simultaneous observation of different systems depending on their fluorescent label.
During an FCS experiment, fluorescent systems are typically excited by continuous wave excitation. After the record of the fluorescence intensity fluctuations a correlation analysis is applied. A continuously fluctuating fluorescence signal can be identified as a steep decrease in a correlation curve.[54]Intensity fluctuations within fluorescent systems can be observed for example during chemical reactions, photophysical transformations or conformational changes. The obtained correlation time is connected to the respective chemical rate or diffusion constants.
The combination of TCSPC and FCS leads to fluorescence lifetime corre-lation spectroscopy (FLCS).[55,56]Here, a separate correlation analysis for each recorded fluorescence lifetime component is conducted, which can be detected from several fluorescent species within the sample. The setup for FLCS resembles the experimental setup for FCS and is shown in Figure 2.8.
It consists of a confocal microscope equipped with a TCSPC device and an FCS device. A repetitive laser pulse is focused by an objective lens through a small confocal volume through the fluorescent sample. The emitted light is again collected by the objective and separated from the excitation laser beam using a dichroic mirror. The fluorescent beam is collimated through a micrometer-sized pinhole and focused onto two single-photon avalanche photodiodes using a beam splitter.
Figure 2.8:Schematic depiction of a setup for fluorescence lifetime correlation spec-troscopy.[54]Adapted with permission from A. Ghoshet al., Methods2018,140-141, and used without modification.
FLCS is an extended FCS method and can be applied when information about specific fluorescence lifetimes or fluctuations of fluorescence lifetime within the system is required.
Metal-induced energy transfer
The fundamental phenomenon behind metal-induced energy transfer (MIET) is theFörsterresonance energy transfer (FRET).[57] FRET is based on the non-radiative energy transfer from an excited fluorescent donor to an ab-sorbing acceptor molecule. Theodor Förster illustrates in his publication from 1948 that the efficiency of energy transfer depends on the sixth power of the donor–acceptor distance.[58]This distance dependence made FRET a powerful tool within fluorescence analytics to measure distances on the molecular scale up to approximately 10 nm.[57]
The replacement of the acceptor molecule with a metal surface showed a similar effect to the phenomenon of FRET.[59]However, the plane metal sur-face quenches the donor’s fluorescence emission and, therefore, decreases the fluorescence lifetime. Since the energy transfer rate depends on the distance between the fluorescent system and the metal surface, the distance value can directly be obtained by converting the fluorescent lifetime.[57]
Equivalent to FRET, the metal-induced energy transfer also is a near-field effect. However, the relation between fluorescence lifetime and distance ranges from zero to 100−200 nm due to the planar geometry of the metal surface.
The phenomenon of the metal-induced energy transfer is mediated by an electromagnetic field, which originates from an oscillating electric dipole.[60]
Nearly every organic fluorescent dye can be considered as an ideal electric dipole oscillator. The fluorescence lifetime depends on the orientation of the dipole to the metal surface. This dependence is shown in Figure 2.9.[57]
Figure 2.9:Illustration of calculated dependence of the relative fluorescence lifetime to the distance of the fluorescent dye to the metal surface. The fluorescent dye is considered as a oscillating dipole. The orientation of the dipole influences the lifetime–
distance dependence.[57]Adapted with permission from A. Chizhiket al., Nature Photonics2014,8, and used without modification.