**1. Introduction**

Aggregation structures such as those of hydrophobic and hydrophilic surfactant molecules widely exist in aqueous solutions, micelles, liquid crystals, various membranes, and biological systems and are important for understanding physical and chemical properties and functions. Extensive investigations have demonstrated that optic responses of molecular aggregates

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

often determine the property and performance of optical functional materials [1–3]. Also found was that, selective controlling of excitonic states of molecular aggregates profits to engineer optical properties of promising photonic materials described in terms of the model of Frenkel excitons [4–6]. In poor solvents, self-assembly of organic molecules in the form of weakly coupled aggregates display significantly different spectroscopic behavior compared to their monomers [7]. From UV-vis spectral analysis based on Kasha exciton theory [8–10], typical aggregation behavior can be determined by checking out a tilt angle of molecular stacking [11–13]. In addition to UV-vis absorption, fluorescence of aggregates has also been meticulously investigated, where the fluorescence intensity of organic molecules often diminishes upon aggregation due to intermolecular interactions [14–18], but allowing for interesting exceptions such as those demonstrated as aggregation-induced emission enhancement (AIEE), revealing relationships between molecular structures/molecular arrangements and emission properties [2, 3, 19–21]. While numerous UV-vis and fluorescence investigations help determine the aggregation behavior, it is important to utilize vibrational spectroscopic fingerprints of molecular aggregates to identify the components and structures, phase transition, likely isomers, and conformation transition of chemicals at reduced sizes [22, 23].

Raman effect arises when light impinges upon a molecule or molecule aggregates and interacts with the electron cloud and chemical bonds. A fascinating world of Raman spectroscopy toward both bulk materials and single molecules has been fully demonstrated (i.e., the two ends), but for molecule aggregates, there are relatively less Raman spectroscopic studies in reported publications so far. A few Raman investigations have shed light on ionic surfactants [24–28] and photogenerating reactions [29–31] of organic molecule assemblies; and in-situ Raman techniques have been utilized to monitor real-time reactions and catalysis [32–36], as well as photo-induced polymerization and magnetization [37]. Recently, it was reported that small organic molecules could form uniform assembly with head-to-tail J-aggregation along the inner walls of the pores of an anodic aluminum oxide (AAO) template, giving rise to an interesting topic for surface-enhanced Raman spectroscopy [11, 13, 38]. Also, there were a few surface-enhanced Raman spectroscopy (SERS) investigations endeavoring to determine the plasmonic property of metal clusters and structural information of molecule aggregates. Raman spectroscopic studies of atomic/molecular clusters and aggregates are expected to become a significant solution to identify chemical structures at primary state of nucleation and growth of materials, gas-to-particle conversion mechanism of aerosols, as well as the aggregation states in liquid crystals, micelles, bilayers/monolayers, and biomembranes. These efforts also help understand the fundamentals in various fields such as catalysis, optics, magnetism, and medicine, etc.

#### **2. Raman theory of aggregated molecules**

According to the molecular exciton theory developed by Davydov [9] and Kasha [8, 10], the aggregation of molecules alters their absorption spectrum, reflecting hypsochromic (i.e., blueshift) or bathochromic (i.e., red shift) absorption bands, corresponding to H-aggregate (faceto-face, or side-by-side) or J-aggregate (head-to-tail, or linear herring bone) absorptions, respectively. Assuming that different accumulations of molecules have different tilt angles (defined as the angle between the transition dipole and the molecular axis of the aggregate), typical H-aggregates bear a tilt angle value greater than 54.7° and exhibit a broader, blueshifted absorption band, while J-aggregates bear a tilt angle smaller than 54.7° characterized by a red-shift in the UV-vis spectrum relative to the monomer [11–13]. From UV-vis spectral analysis based on Kasha exciton theory [8–10], simply the approximate tilt angle for accumulation of N molecules can be calculated according to the following equation,

often determine the property and performance of optical functional materials [1–3]. Also found was that, selective controlling of excitonic states of molecular aggregates profits to engineer optical properties of promising photonic materials described in terms of the model of Frenkel excitons [4–6]. In poor solvents, self-assembly of organic molecules in the form of weakly coupled aggregates display significantly different spectroscopic behavior compared to their monomers [7]. From UV-vis spectral analysis based on Kasha exciton theory [8–10], typical aggregation behavior can be determined by checking out a tilt angle of molecular stacking [11–13]. In addition to UV-vis absorption, fluorescence of aggregates has also been meticulously investigated, where the fluorescence intensity of organic molecules often diminishes upon aggregation due to intermolecular interactions [14–18], but allowing for interesting exceptions such as those demonstrated as aggregation-induced emission enhancement (AIEE), revealing relationships between molecular structures/molecular arrangements and emission properties [2, 3, 19–21]. While numerous UV-vis and fluorescence investigations help determine the aggregation behavior, it is important to utilize vibrational spectroscopic fingerprints of molecular aggregates to identify the components and structures, phase transition, likely isomers, and conformation transition of chemicals at reduced sizes [22, 23]. Raman effect arises when light impinges upon a molecule or molecule aggregates and interacts with the electron cloud and chemical bonds. A fascinating world of Raman spectroscopy toward both bulk materials and single molecules has been fully demonstrated (i.e., the two ends), but for molecule aggregates, there are relatively less Raman spectroscopic studies in reported publications so far. A few Raman investigations have shed light on ionic surfactants [24–28] and photogenerating reactions [29–31] of organic molecule assemblies; and in-situ Raman techniques have been utilized to monitor real-time reactions and catalysis [32–36], as well as photo-induced polymerization and magnetization [37]. Recently, it was reported that small organic molecules could form uniform assembly with head-to-tail J-aggregation along the inner walls of the pores of an anodic aluminum oxide (AAO) template, giving rise to an interesting topic for surface-enhanced Raman spectroscopy [11, 13, 38]. Also, there were a few surface-enhanced Raman spectroscopy (SERS) investigations endeavoring to determine the plasmonic property of metal clusters and structural information of molecule aggregates. Raman spectroscopic studies of atomic/molecular clusters and aggregates are expected to become a significant solution to identify chemical structures at primary state of nucleation and growth of materials, gas-to-particle conversion mechanism of aerosols, as well as the aggregation states in liquid crystals, micelles, bilayers/monolayers, and biomembranes. These efforts also help understand the fundamentals in various fields such as catalysis, optics, magnetism,

and medicine, etc.

32 Raman Spectroscopy and Applications

**2. Raman theory of aggregated molecules**

According to the molecular exciton theory developed by Davydov [9] and Kasha [8, 10], the aggregation of molecules alters their absorption spectrum, reflecting hypsochromic (i.e., blueshift) or bathochromic (i.e., red shift) absorption bands, corresponding to H-aggregate (faceto-face, or side-by-side) or J-aggregate (head-to-tail, or linear herring bone) absorptions,

$$
\Delta \nu = 2 \frac{N-1}{N} \cdot \frac{}{h \cdot r^3} (\text{l} - 3 \cos^2 a) \tag{1}
$$

$$
\left\langle \mathbf{m}^2 \right\rangle = 9.185 \times 10^{-39} \int\_{\lambda\_1}^{\lambda\_2} \varepsilon(d\lambda \, / \, \lambda) \tag{2}
$$

where Δν is the spectral shift from the monomer absorption; *h* is the Planck's constant; *r* is the separation of centers; *α* is the tilt angle between the line of center and molecular long axes; <*m*<sup>2</sup> > refers to the transition dipole moment of monomer; *ε* aims at the molar extinction coefficient in (moles/L)−1 cm−1; *λ* is the wavelength; and *λ*1 and *λ*2 are the limits of a well-defined absorption band. This theory has been successfully applied to determine the J-aggregation of small organic molecules, such as perylene [11], assembled on pipe inner wall of AAO templates. This is further discussed below.

Further, in light of the molecular exciton theory, Akins [39] reported a study on Raman scattering enhancement theory for a finite aggregate structure consisting of N molecules, assuming the formation of molecular vibro-excitonic levels. The quantum mechanical Hamiltonian describing the internal system of the N molecules can mutually interact through a potential term V, which was given by [39],

$$\mathbb{H} = \sum\_{n=1}^{N} \left( H\_n + \sum\_{m>n} V\_{nm} \right) \tag{3}$$

where *H* is the Hamilton operator of kinetic energy; and *Vnm* refers to the interaction potential of molecules *n* and *m* (*m* is any of the other molecules taken together with molecule *n*). Thus for an individual molecule *n*, the vibronic wave function can be given by [39],

$$
\varphi\_{nj}(\rho, \mathbf{Q}) = \varphi\_n(\rho, \mathbf{Q}\_0) \mathbf{x}\_{ni}(\mathbf{Q}) \tag{4}
$$

here *n* aims at the position of the probe molecule and *j* refers to a composite quantum number corresponding to the number of vibrational quanta of excitation. The character *ρ* is a composite spatial coordinate of the electrons, *Q* is the normal coordinate, and *Q*<sup>0</sup> represents the normal coordinates for the ground-state equilibrium configuration.The function *ϕ* refers to a multiple electron wave function depending on the coordinates and spins of the electrons, while *x* corresponds to the vibrational wave function. Based on Born-Oppenheimer approximation, the normalized vibro-exciton wave function can be ascertained and thus the allowed energies. Further, according to the theory by Craig and Thirunamachandra [40], the Raman scattering intensity in a particular direction can be given by [39],

$$I\left\{\boldsymbol{k}'\right\}=\frac{\tilde{\mathcal{N}}\boldsymbol{I}\_{\boldsymbol{k}'}^{4}}{16\pi\varepsilon\_{0}\,^{2}}\bigg|\sum\_{\boldsymbol{r}}\left\{\frac{\left(\tilde{\boldsymbol{\mu}}^{mr}\cdot\vec{\boldsymbol{e}}'\right)\left(\tilde{\boldsymbol{\mu}}'^{0}\cdot\vec{\boldsymbol{e}}\right)}{E\_{r0}-\hbar ck}+\frac{\left(\vec{\boldsymbol{\mu}}^{mr}\cdot\vec{\boldsymbol{e}}\right)\left(\tilde{\boldsymbol{\mu}}'^{0}\cdot\vec{\boldsymbol{e}}'\right)}{E\_{r0}+\hbar ck}\right\}\bigg|\tag{5}$$

where *Ñ* is the number of scattering centre; is transition dipole moment vector; is microscopic electric field strength vector; and *E*r0 shows the energy difference between the upper excited state r and the ground state 0. From first-order perturbation theory, the Raman scattering of aggregated molecules can be expressed as [39],

$$
\wedge\_{rs}(Q) = (\sum\_{a} h\_{rs} \, ^a Q\_a)(\Delta E\_{rs} ^0)^{-1} \tag{6}
$$

in which *hrs <sup>α</sup>* is a coupling term between electronic states r and s for the molecule with equilibrium ground-state configuration [39]. This theory has been successfully applied to dye molecules that form ground-state and excited-state aggregation structures, where an intrinsic enhancement was often gained on the formation of aggregates containing N monomers [39, 41].

#### **3. Plasmon-free Raman scattering of molecule aggregates**

Raman scattering of molecule aggregates, held together by dispersion and electrostatic forces, has been found different from that of non-aggregated monomers and bulk crystals. Moreover, the Raman bands observed for the aggregates could shift from those of the isolated monomers depending on the intermolecular interactions (i.e., strong or weak; covalent or non-covalently coupled interactions). This characteristic enables Raman spectroscopic studies of molecule aggregates to determine phase transition and photo-assisted polymerization. It is worth noting that, upon resonant excitation, the excited states often bring forth new Raman bands associated with lattice motions, e.g., typical motions in the aggregate formation direction. In this section, we emphasize on a few examples of normal Raman investigations of molecule aggregates.

#### **3.1. To determine phase transition**

It has been widely recognized that the assembly of target molecules can result in novel responses of Raman spectroscopy. For example, when examining Raman spectra of barium dialkyl phosphates at various chain lengths, Okabayashi et al. [42] found that the PO2 − symmetric stretching mode of dipentyl phosphate appears at 1106 cm−1 for the liquid crystal state formed at room temperature, which differs from that in the aqueous solution (ca. 1075 cm−1). Also found was that, the relative intensities of the Raman spectral lines changed sharply at the phase-transition temperature but were found to be a constant below and above the transition point. By fully examining the Raman intensities and Raman shifts of the PO2 − symmetric-stretching modes of dipentyl phosphate (1075 cm−1), diester O-P-O and dibutyl phosphate (at 1090 and 1068 cm−1), phase transition and the coexistence of two types of aggregation structures were determined. These Raman investigations illustrated how cation-phosphate interactions are important to form aggregation structures and affect the phase transition in liquid crystals of dialkyl phosphates.

#### **3.2. Aggregation-enhanced Raman scattering (AERS)**

electron wave function depending on the coordinates and spins of the electrons, while *x* corresponds to the vibrational wave function. Based on Born-Oppenheimer approximation, the normalized vibro-exciton wave function can be ascertained and thus the allowed energies. Further, according to the theory by Craig and Thirunamachandra [40], the Raman scattering

( ) ( )( ) ( )( ) <sup>2</sup> 0 0 <sup>4</sup>

% *mr r mr r*

ì ü × × ×× ï ï = + í ý

*r r r*

where *Ñ* is the number of scattering centre; is transition dipole moment vector; is microscopic electric field strength vector; and *E*r0 shows the energy difference between the upper excited state r and the ground state 0. From first-order perturbation theory, the Raman

equilibrium ground-state configuration [39]. This theory has been successfully applied to dye molecules that form ground-state and excited-state aggregation structures, where an intrinsic enhancement was often gained on the formation of aggregates containing N monomers [39, 41].

Raman scattering of molecule aggregates, held together by dispersion and electrostatic forces, has been found different from that of non-aggregated monomers and bulk crystals. Moreover, the Raman bands observed for the aggregates could shift from those of the isolated monomers depending on the intermolecular interactions (i.e., strong or weak; covalent or non-covalently coupled interactions). This characteristic enables Raman spectroscopic studies of molecule aggregates to determine phase transition and photo-assisted polymerization. It is worth noting that, upon resonant excitation, the excited states often bring forth new Raman bands associated with lattice motions, e.g., typical motions in the aggregate formation direction. In this section, we emphasize on a few examples of normal Raman investigations of molecule aggregates.

It has been widely recognized that the assembly of target molecules can result in novel responses of Raman spectroscopy. For example, when examining Raman spectra of barium dialkyl phosphates at various chain lengths, Okabayashi et al. [42] found that the PO2


*<sup>α</sup>* is a coupling term between electronic states r and s for the molecule with

h

(6)

−

*πε E ck E ck'* (5)

r r r r r rr r

<sup>0</sup> 0 0 16

*NI μ e' μ e μ e μ e' I k*

intensity in a particular direction can be given by [39],

34 Raman Spectroscopy and Applications

2

scattering of aggregated molecules can be expressed as [39],

**3. Plasmon-free Raman scattering of molecule aggregates**

in which *hrs*

**3.1. To determine phase transition**

*k'*

¢ å <sup>h</sup>

A few meticulous investigations dealing with relative Raman intensities and selection rules for aggregates have demonstrated a concept termed "aggregation-enhanced Raman scattering" (AERS) [43], which was proposed to represent a concept solely for studies of aggregates, which differs from a mechanism based on SERS, resonance Raman scattering, and Mie scattering since these Raman effects ignore the impact of aggregation of molecules. The aggregation of molecules in their ground state may result in enhanced polarizability compared to monomeric species and hence intensified radiation according to the basic principle for a dipole moment *μ* = *αE*, where *α* is the polarizability and *E* is the coupled field. On the other hand, the aggregates could form molecular excitonic states allowing for a coupling effect (between excitonic states) which alters the cross-section of Raman scattering, because the participation of more energetic states readily gives rise to an enhanced response to incident radiation [43]. In this point, resonance Raman scattering, where the incident exciton overlaps a small number of exciton bands, is expected to result in further enhancement of vibrational bands. Akins have conducted numerous investigations relating to AERS [41, 44–54], as partly included in a recent review article [43].

#### **3.3. Resonance Raman effect from aggregation**

Resonant Raman spectroscopy (RRS) has been known as a main enhancement strategy to solve the sensitivity issue and to derive Raman labels for applications. For example, Zajac et al. [23] reported an interesting study on aggregation-induced resonance Raman optical activity (AIRROA) of astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione, AXT), a chiral xanthophyll that was known to bear high antioxidant potency beneficial to cardiovascular, inflammatory, immune and neurodegenerative diseases. Along with striking differences of UV-vis spectra of the monomer and aggregates, Raman spectra (**Figure 1**) demonstrated spectral changes (e.g., 5 cm−1 blue-shift of the C═C stretching mode, and the appearance of a new band at ~280 cm−1) due to J-aggregation of astaxanthin along with a quenched fluorescence background. Considering the fact of low sensitivity determined by the main limitation of Raman optical activity (c.a., approximately one photon in a billion), resonance effect and aggregation enhancement is important to further advances of AIRROA investigations.

**Figure 1.** (A) A schematic representation of the 6T@f-SWNT with bromophenyl groups grafted onto its sidewall, and description of the encapsulation and chemical functionalization steps to prepare the α-sexithiophene encapsulated inside a covalently functionalized SWNT (6T@f-SWNT). (B) Raman spectra at *λ* = 532 nm excitation of an individual SWNT, α-sexithiophenes (6T) inside a SWNT (6T@SWNT) and after the covalent functionalization step (6T@f-SWNT). The polymer monoliths composed Ref. [55].

Recently, Gaufrès et al. [55] reported an interesting study on encapsulated and aggregated dye molecules inside single-walled carbon nanotubes [55], where giant Raman scattering effect was discovered. Raman measurements for the rod-like dyes (α-sexithiophene and β-carotene) assembled in single-walled carbon nanotubes exhibit highly-polarizable J-aggregates, as shown in **Figure 1**, giving an enhanced resonant Raman cross-section above that required for detecting individual aggregates. It was found that the shielding of carbon nanotube enables fluorescence-free background and photobleaching-free Raman signals, allowing the giant Raman effect used as functionalized nanoprobe labels for Raman imaging with robust detection using multispectral analysis. Beside this, there are also a few other interesting research papers dealing with Raman scattering by encapsulated molecules in carbon nanotubes, where the formation of aggregates give rise to well-resolved Raman spectra due to interaction and charge transfer within the carbon nanotubes [56, 57].

#### **3.4. Magnetic field-trapped Raman scattering**

The exploration of magnets and magnetism is associated with human history. Recently, Luo et al. [37] reported an interesting photo-assisted method to magnetize microcrystal fullerene C60 at room temperature by exciting C60 molecules to triplet states via proper laser radiation and then trapping the spin-polarized states under a strong magnetic field (**Figure 2**). Raman spectroscopy was found an operative probe due to its fingerprint spectra regarding energy levels and molecular states, and the crystalline form of C60 molecule aggregates is held together by van der Waals forces allowing the conversion to polymeric phase under proper laser radiation [58]. As results, novel changes on Raman scattering of micro-crystal solid C60 were discovered in the presence and absence of the magnetic field; also found was that the Raman spectra exhibited "hysteresis" phenomenon when the external magnetic field was removed. Together with first-principles calculations which well reproduced the Raman activities of C60 on different states [37], as seen in **Figure 2**, photo-assisted magnetization (PAM) of the fullerenes and magnetic-field trapped Raman spectroscopy (MFTRS) were proposed [37]. The PAM strategy with MFTRS verification opens a new approach and, as a general protocol, enables the magnetization of common materials that consist of only light elements. The importance of spin-spin and spin-orbit interactions was also demonstrated in nano-graphene fragments [59]; Raman spectroscopy also plays an important role in identifying single- and few-layer graphene [60, 61].

**Figure 2.** (A) Calculated Raman activity of a singlet-state C60 monomer (a), comparison of a normal FT-Raman spectrum of solid C60 (b); (B) calculated Raman spectrum of a quintet-state dimer C60 (a), compared with the 514.5-nm Raman of solid C60 in the presence of a *B* = 2.5 T magnetic field (b); (C and D) The strongest vibrational modes of a singletstate C60 monomer (1466-cm−1) and a quintet-state C60 dimer (1458-cm−1). The displacement vectors are shown with red arrows.

#### **3.5. Raman probes for aerosols**

**Figure 1.** (A) A schematic representation of the 6T@f-SWNT with bromophenyl groups grafted onto its sidewall, and description of the encapsulation and chemical functionalization steps to prepare the α-sexithiophene encapsulated inside a covalently functionalized SWNT (6T@f-SWNT). (B) Raman spectra at *λ* = 532 nm excitation of an individual SWNT, α-sexithiophenes (6T) inside a SWNT (6T@SWNT) and after the covalent functionalization step (6T@f-SWNT).

Recently, Gaufrès et al. [55] reported an interesting study on encapsulated and aggregated dye molecules inside single-walled carbon nanotubes [55], where giant Raman scattering effect was discovered. Raman measurements for the rod-like dyes (α-sexithiophene and β-carotene) assembled in single-walled carbon nanotubes exhibit highly-polarizable J-aggregates, as shown in **Figure 1**, giving an enhanced resonant Raman cross-section above that required for detecting individual aggregates. It was found that the shielding of carbon nanotube enables fluorescence-free background and photobleaching-free Raman signals, allowing the giant Raman effect used as functionalized nanoprobe labels for Raman imaging with robust detection using multispectral analysis. Beside this, there are also a few other interesting research papers dealing with Raman scattering by encapsulated molecules in carbon nanotubes, where the formation of aggregates give rise to well-resolved Raman spectra due to

The exploration of magnets and magnetism is associated with human history. Recently, Luo et al. [37] reported an interesting photo-assisted method to magnetize microcrystal fullerene C60 at room temperature by exciting C60 molecules to triplet states via proper laser radiation and then trapping the spin-polarized states under a strong magnetic field (**Figure 2**). Raman spectroscopy was found an operative probe due to its fingerprint spectra regarding energy levels and molecular states, and the crystalline form of C60 molecule aggregates is held together by van der Waals forces allowing the conversion to polymeric phase under proper laser radiation [58]. As results, novel changes on Raman scattering of micro-crystal solid C60 were discovered in the presence and absence of the magnetic field; also found was that the Raman

interaction and charge transfer within the carbon nanotubes [56, 57].

**3.4. Magnetic field-trapped Raman scattering**

The polymer monoliths composed Ref. [55].

36 Raman Spectroscopy and Applications

Raman spectroscopy is useful in characterizing atmospheric aerosols profiting from the development of portable Raman instrument in recent years [62–77]. For example, Aggarwal et al. [77] developed a Raman spectrometer using 532-nm continuous wave laser and used for detecting and identifying chemical aerosols of a low-concentration in atmospheric air. As results, they demonstrated the successful application of Raman for trace detection and analysis of iso-vanillin aerosols up to a mass concentration of 1.8 ng/cm3 with the signal-to-noise ratio at about 19 in 30s for the 1116-cm−1 mode with a decent Raman cross section of 3.3 × 10−28 cm2 at the use of 8-W double-pass laser power. Among others, Batonneau et al. [78] reported an interesting study on heterogeneous chemistry of aerosol particles utilizing Raman mapping and spectroscopic method, which was found in agreement with elemental images obtained by X-ray-mapping. An et al. [79] conducted a systematic study to identify a few typical organic compounds (isoprene, terpenoids, pinenes etc.) which are known as the main sources of organic aerosols (OAs) particle matter in air pollution. Raman and IR spectra of isoprene, terpenoids, pinenes and their mixtures were examined showing distinguishable vibrational spectroscopic fingerprints of the three components respectively. It was noted that, in a certain case such as β-pinene, a dimer model reproduces the experimental results other than single molecule modeling, indicating nonneglectable intermolecular interactions and aggregation states for aerosols challenging the present mechanisms based on single molecule theory. Further, Raman spectra from an ambient sample can be analyzed using a hierarchical clustering method to check out whether the spectra of aerosols in consistence with relating organic compounds. In particular, analysis on time-resolved aerosol Raman spectra over the course of several hours, simply by checking the D-G bands of amorphous carbon plotted vs time (e.g., a half-hour intervals), enables to monitor and judge the increase/decrease of related pollution in atmosphere [78, 80, 81].

**Figure 3.** (Top) Standard optical tweezers (Biral AOT 100) arrangement. (Inset) Valve system used to initiate exchange between D2O and H2O. (Bottom) A sketch showing isotopic water diffusion in aerosol by the use of optical tweezers. Reproduced with permission from Ref. [71].

Recently, Davies and Wilson [71] employed an aerosol optical tweezer technique for contactless levitation of single droplets (e.g., 3–6 μm in radius) and then for Raman investigations, as shown in **Figure 3**. Flexible environmental control system allows for rapid exchange of the gas-phase humidity source between H2O and D2O (**Figure 3**) to monitor the progression of the droplet composition using Raman spectroscopy. Utilizing a model describing diffusion in a sphere (i.e., solution to Fick's second law), they analyzed the data by varying diffusion coefficients (*D*w) in viscous media to achieve the best fit to both D2O and H2O data sets. This droplet-based isotopic tracer method takes a few advantages for measurement of diffusion coefficients. The resolution of gel formation suggests promising application to identify phase behavior that leads to abrupt changes in water mobility (e.g., hydrophobic phase separation, aerosol formation and rapid growth), enabling to explore the changing role of water diffusion at chemical transformation thus valuable insights into the oxidative aging behavior in determining diffusive properties of atmospheric aerosol.
