The fate of melanin in the epidermis is of good interest due to its joining in many physiological and pathological processes in the skin. Melanin localization deserve to be assessed ex vivo and also in vivo making use of its distinctive optical properties. Melanin exhibits a characteristic Raman spectrum band shape and discernible near-infrared excited (NIR) fluorescence. However, a detailed evaluation of the ability of depth-resolved confocal Raman and fluorescence microspectroscopy in the testimonial of melanin distribution in the human skin is lacking. Below we demonstrate how the portion of melanin at various depths in the human being skin in vivo can be estimated from that Raman spectra (bands in ~ 1,380 and 1,570 cm−1) using several procedures consisting of a simple ratiometric approach, spectral decomposition and also non-negative procession factorization. The depth file of matrix factorization components details to melanin, collagen and also natural moisturizing factor administer information around their localization in the skin. The depth profile of the collagen-related matrix factorization component allows for precise determination the the dermal–epidermal junction, i.e. The epidermal thickness. Spectral features of fluorescence lift originating from melanin were discovered to correlate with relative intensities the the melanin Raman bands. We likewise hypothesized that NIR fluorescence in the skin is no originated solely from melanin, and also the feasible impact the oxidized types should it is in taken into account. The proportion of melanin-related Raman bands in ~ 1,380 and also 1,570 cm−1 can be pertained to melanin molecular organization. The proposed merged analysis that the Raman scattering signal and NIR fluorescence can be a useful tool for rapid non-invasive in vivo diagnostics the melanin-related procedures in the human skin.

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Melanin, the pigment mostly responsible for skin color, participates in plenty of physiological processes. Melanin exhibits photoprotective, antioxidative, and, in part cases, photosensitizing properties1. That is likewise directly affiliated in the life bike of the healthy and pathological epidermis, consisting of such a significant malignancy together melanoma2. Being heterogeneous end-products the the complicated transformation that L-tyrosine3, melanins deserve to be classified right into several chemically distinctive subtypes, of i m sorry eumelanin (black and also brown) and also pheomelanin (yellow-reddish) are current in the human being epidermis4,5. In general, the framework of organic melanin is a facility issue, together natively arising melanin is a mixture of various pigment subtypes produced from the very same precursor together a result of “mixed melanogenesis”6.

Melanin is developed in melanocytes—the cells, which at normal conditions are localized in the basal great of the epidermis. Packed right into melanosomes, melanin is transferred to the adjacent keratinocytes: ~ above average, a solitary melanocyte and ≈ 30–40 bordering keratinocytes write an epidermal melanin unit, and this ratio is maintained in various skin phototypes7. The subtype that melanin (ratio the eu- and pheo-melanin) and amount of melanin, and also the amount, pack degree and size of the melanosomes contribute to the formation of the skin color. However, a finish understanding of melanin carry mechanisms, that further distribution in the epidermis and also degradation pathways stays elusive. Because that instance, melanin circulation was considered to be regulation by melanosomes degradation via keratinocytes’ autophagic activity8. However, in later on works, autophagy was demonstrated not to have actually a comprehensive effect ~ above skin pigmentation9, and also melanosomes were shown to it is in non-degradative10. Other mechanisms like inherent asymmetric localization the melanosomes in between dividing keratinocytes might be potentially associated in a complicated inhomogeneous circulation of melanin in the epidermis11,12,13. Hence, specific assessment of melanin circulation in different epidermal great is required to study fundamental processes and also identify melanin-related abnormalities in norm and pathology.

A standard device for melanin visualization in the skin is the histochemical staining. This procedure, however, is vulnerable to false-positive results: because that instance, observation of “melanin dust” (degraded melanin fragments) in the stratum corneum through this approach was displayed to be an artifact brought about by non-specific reduction of silver by non-melanin compound in the skin13,14. The use of various other staining agents, e.g. Based upon immunohistochemistry, permits for details labeling of melanogenesis- and melanosome-specific proteins, however these techniques are also highly labile14. A common disadvantage the histological researches is its invasiveness, i m sorry imposes restrictions on the number of patients connected in the investigation, the forced time that analysis, and also the i can not qualify to examine the same area that skin lot of times. Hence, optical approaches have emerged as a new generation of techniques for non-invasive diagnostics of the skin. In this regard, the unique optical nature of melanin made it a perfect target for ex vivo and in vivo imaging (Table 1).

Table 1 an overview on non-invasive optical methods used because that quantification, imaging and also structural characterization of melanin in the skin
Full dimension table

Unlike the other endogenous chromophores in the skin, melanin exhibits a certain broadband absorption, which decreases tremendously from short (UV) towards lengthy (NIR) wavelengths. The long-wavelength absorb is attach by a discernible red/NIR excited fluorescence, and also melanin is considered to it is in the significant source that NIR fluorescence in the skin. That is, quantification of melanin in the skin deserve to be performed by single-photon NIR-excited fluorescence imaging15,16,17,18,19,20. Moreover, melanin fluoresces ~ above two-photon excitation, allowing for its in vivo assessment and 3D imaging21,22. However, the fluorescence intensity chin does not provide details information top top the melanin type and that environment23,24. Added contrasting the melanin can be accomplished using fluorescence lifetime imaging microscopy (FLIM), together melanin exhibits a distinctive quick (25,26,27. Next, using the pump-probe imaging technique, one might differentiate between eumelanin and also pheomelanin in ~ the dermal–epidermal junction to evaluate the metastatic potential that melanin lesions and characterize the pack of melanin oligomers within the aggregates28,29,30,31. Other optical techniques, including reflectance confocal laser scanning microscopy32,33,34 and optical coherence tomography35,36 may aid in the detection the pathological melanocytic lesions. The long-wavelength absorption tail also enables the use of the photoacoustic signal come quantify melanin in the skin37,38,39.

Melanin exhibits a characteristics Raman spectrum. Lot of bands may add to the formation of the Raman spectrum the melanin in the 1,000–1,800 cm−1 range: the tape at 1,220 cm−1, which corresponds to the stretching vibrations of the phenolic C–OH and C–O stretching in carboxylic acids, the 1,340 cm−1 tape of the C–N extending of the indole, the band through a maximum in ~ 1,390 cm−1 developed by the C=C “breathing” vibrations that the fragrant structures (A1g symmetry, similar to the D-band in disordered graphite). Also, the 2 bands at 1562 and also 1598 cm−1, attributed to the stretching mode of sp2 hybridized carbon the C=C and also E2g setting of C–C vibrations that the fragrant ring in the indole framework of eumelanin, add to the it was observed Raman spectrum40. Regardless of this large number that bands, they space poorly resolvable because of the heterogeneous of melanin, and also their superposition results in just two vast bands with maxima at ≈ 1,380 and also ≈ 1,570 cm−1 and FWHMs of ≈ 200 and also 150 cm−1, respectively. Relying on the excitation wavelength, the structural arrangement of melanin and also its biochemical environment, the amplitude and also positions of these bands may vary, enabling the characterization that melanin in vitro and also quantification of melanin in vivo40,41,42,43,44.

In this work, we concentrated on a an unified application the confocal Raman and also fluorescence microspectroscopy to determine the depth-resolved melanin content in the person skin in vivo. Currently, a detailed analysis of the relationship in between NIR-excited fluorescence and also the molecular properties the melanin, i m sorry are shown up in the Raman spectra, is doing not have in the literature. Hence, us made usage of confocal Raman microspectroscopy come disentangle the impacts of fluorescence and Raman signals from melanin in order to investigate their diagnostic capabilities and also studied the interconnections in between the attributes of the Raman and fluorescence spectra that melanin in the human being skin in vivo.


A total of 10 healthy and balanced volunteers (2 male, 8 female) aged from 22 come 60 years (average 37.5 years) v skin type II (5 participants) and also III (5 participants) according to the Fitzpatrick group were associated in the study. All volunteer did no use any skincare products on the inside forearms at least 72 h and did not take a bath or shower at the very least 4 h before the beginning of the experiments. Ripe volunteers connected in the research reported the lack of sunbathing at the very least 3 months before the experiment; the volunteers through a high portion of melanin were naturally tanned. One volunteer (skin form III) experienced sun exposure for 3 days prior to the measurements.

All volunteers had offered written educated consent. A hopeful vote has been obtained from the ethics committee that the Charité – Universitätsmedizin Berlin and also the experiments were in accordance through the principles of the declaration of Helsinki as revised in 2013.

Confocal Raman microspectroscopy (CRM): measure up parameters

To recognize the possibilities that melanin localization in various epidermal layers, 6 depth-resolved Raman spectra (from 0 come 60 µm depth v increments that 2 µm) to be measured for each that the 10 volunteers in vivo top top the volar forearm using the confocal Raman microscopy (CRM) instrument (Model 3510 SCA, RiverD, Rotterdam, The Netherlands). CRM permitted measuring Raman and also fluorescence signals at the same time from the skin v a spatial resolution of less than 5 µm. The spectra were tape-recorded in the fingerprint (FP: 400–2,200 cm−1) and also high wavenumber (HWN: 2,500–4,000 cm−1) regions upon excitation at 785 (20 mW) nm and also 671 nm (17 mW), respectively. The acquisition times for the FP and HWN spectra to be 5 and also 1 s, resulting in 3 J/cm2 and 0.5 J/cm2 irradiation dose per a solitary measurement, respectively. The major scheme of the utilized CRM might be uncovered elsewhere45.

The skin surface place was determined as the position where the soot of the Amide ns band calculated as the mean intensity in the 1,550–1,720 cm−1 selection after fluorescence background subtraction comparable to the procedure reaches fifty percent of its maximum together it to be proposed in the job-related of Choe et al.46. All data evaluation was performed using a custom-made program based upon the Python programming language v Matplotlib, NumPy, Pandas, Scikit-learn, SciPy libraries.

Fluorescence elevator subtraction

Since melanin is taken into consideration to it is in the significant contributor come the red/NIR-excited fluorescence in the person epidermis, the depth distribution of NIR fluorescence intensity was evaluated. To quantify the intensity and spectral properties of fluorescence emission and also to extract the Raman signal, fluorescence background was evaluated by installation the spectrum with a 2nd order polynomial duty in the FP range, where the 620–700 cm−1 and also 1900–2,200 cm−1 arrays were provided for the interpolation that the fluorescence spectrum due to the absence of intense Raman bands. One analogous approach was previously used to assess and also subtract the fluorescence background in the FP range in spectra v high melanin content41. The choice of 2nd order polynomial role as an estimator for fluorescence background is determined by the fact that ~ above the one hand, it provides reasonable top quality of fit and, ~ above the other, gives a simple estimation ~ above spectral attributes of fluorescence. Interpolation the the fluorescence background in the HWN selection was made using piecewise-weighted-least squares fitting method, whereby 2,600–2,810 cm−1 and 3,800–3,900 cm−1 arrays were used for fluorescence estimation, proposed in47.

Spectra normalization

To change for the fact that intensity of both Raman and also fluorescence signal is attenuated as result of scattering and also absorption that the optical signal by organization with rise of scanning depth, we furthermore normalized the Raman spectra obtained after background individually by the average protein-related Raman intensities in 1,550–1,720 cm−1 and 2,800–3,000 cm−1 ranges, respectively. This normalization procedure is used to monitor concentrations of miscellaneous skin compounds family member to the concentration of proteins both approximated by intensities of Raman bands and also was provided in investigations of the dermis48.

Influence of fluorescence photobleaching

The fluorescence intensity is susceptible to photobleaching, therefore its signal might reduce over time. Together the Raman bands are not topic to photobleaching result in comparison come fluorescence49, the ratio between fluorescence and Raman band intensities might readjust over time. However, according to49 in ~ skin depths of 20–50 µm mean fluorescence decay lifetime top top bleaching is the the order of 30 s because that excitation at 785 nm and also 22.0 mW laser power. For this reason we hypothesize, that 1 and 5 s exposure time walk not impact fluorescence strongness significantly.

Evaluation the NIR fluorescence of the person skin in vivo

Figure 1A,B show the Raman-normalized spectra measured in vivo in the person skin that one volunteer (skin kind II) in ~ the depths varying from 0 come 60 µm v 2 µm increments. Together it have the right to be seen, the Raman signal is superimposed v a fluorescence background in ~ all depths both in the FP (Fig. 1A) and also HWN (Fig. 1B) areas that is typical for skin and biological tissues23.


Characteristic depth-resolved Raman spectra of skin in vivo and examples of fluorescence lift subtraction. (A,B) Depth-resolved Raman spectra the the human being skin in the 400–2,200 cm−1 (A) and 2,500–4,000 cm−1 (B) ranges. Spectra space color-coded according to the measure depth, varying from 0 come 60 µm through 2 µm increments. (C,D). The examples of fluorescence background individually (red dashed line) because that representative Raman spectra (blue line) in the 400–2,200 cm−1 (C) and also 2,500–4,000 cm−1 (D) ranges. Red and blue areas filled in (C,D) stand for fluorescence and Raman signal intensities provided to calculation normalized fluorescence soot (Fl-FP and also Fl-HWN). (E) Correlation in between the normalized fluorescence Fl-FP and also Fl-HWN excited at 785 and also 671 nm, correspondingly.

Figure 1C,D demonstrate common examples that the fluorescence background subtraction in the FP (Fig. 1C) and HWN (Fig. 1D) regions. The fluorescence signal average over the 1,550–1,720 cm−1 (FP region, red area in Fig. 1C) and also 2,800–3,000 cm−1 (HWN region, red area in Fig. 1D) to be normalized by the average Raman signal in the exact same wavenumber areas (blue area in ~ Fig. 1C,D). The described averaged fluorescence intensities in FP and also HWN normalized by Raman signal were more used for subsequent analysis, and hereinafter will certainly be referred as Fl-FP and Fl-HWN.

It to be observed that the calculated normalized fluorescence Fl-FP and Fl-HWN excited at different wavelengths (785 nm and also 671 nm) to be strongly correlated (Fig. 1E, linear approximation R2 = 0.78, p –9), arguing that the exact same fluorophores are responsible because that fluorescence in ~ both excitations. This suggestion is reasonable for melanin, which is defined by the broadband absorption spanning the red/NIR spectral range. From the depth-resolved fluorescence spectra presented in Fig. 1A,B, it deserve to be viewed that the NIR fluorescence signal substantially boosts at ≈ 40 μm depths. This finding likewise suggests that the significant source the the it was observed fluorescence is melanin, i m sorry is typically localized in the basal great (at the depths of about 40 µm because that the volar forearm skin7). Together the fluorescence worths Fl-FP and also Fl-HWN to be strongly correlated, we have actually further focused on the check of the FP region.

Assessment that melanin Raman bands in the skin: spectral decomposition

To assess the affect of melanin come the Raman signal formation, us performed decomposition of the Raman spectra (after fluorescence subtraction) in the 1,200–1,800 cm−1 region using Gaussian functions. Together it have the right to be watched in Fig. 2A,B, the spectral band shape transforms significantly for the spectra with different fluorescence intensity (Fl-FP). The Raman spectra through high fluorescence background (Fl-FP > 15) (Fig. 2A,C) could be nicely described by the pair peaks the melanin through maxima near 1,380 and 1,570 cm−1. Indeed, these spectra are comparable to that of melanin in vitro41 (see the inset in Fig. 2A). In ~ the exact same time, the spectra with low fluorescence background exhibited three major bands at 1,298, 1,450 and 1655 cm−1, mainly corresponding to the vibrations the lipids and proteins that the skin50.


(A,B) common Raman spectra the the person skin in the FP an ar observed because that high (Fl-FP = 26, (A) depth 36 µm) and low (Fl-FP = 3, (B) depth 18 µm) lift fluorescence intensity. The inset in (A) demonstrates the Raman spectrum of fabricated melanin (M8631, Sigma), digitized from41. (C,D). The results of the decomposition that the Raman spectrum in the 1,200–1,800 cm−1 variety for the samples v high (Fl-FP = 26, C) and also low (Fl-FP = 3, D) fluorescence intensity. High intensities that lipid and protein bands at 1,298, 1,450 and also 1655 cm−1 are observed for low fluorescence spectra (D), when the predominance of melanin bands at 1,380 and also 1,570 cm−1 is observed for the spectra v high fluorescence lift (C). Arrowhead in (C) denotes the melanin band situated at ≈ 480 cm−1.

Hence, to identify the melanin content, we decomposed the Raman signal utilizing intense Raman bands the lipids and also proteins in ~ 1,298, 1,450 and 1,655 cm−1 and also two melanin-related bands at 1,380 and 1,570 cm−1. The approximation was performed utilizing multiple Gaussian functions with fixed center positions and FWHMs, so only the amplitudes were varied. The FWHM worths for the Raman bands the proteins and also lipids in ~ 1,298, 1,450 and also 1655 cm−1 were resolved to 80, 45 and also 50 cm−1 correspondingly. Because the melanin-related Raman bands space broadened as result of its heterogeneous, FWHM worths for the bands with centers at 1,380 and 1,570 cm−1 were addressed to 235 and also 165 cm−1, in accordance through the ahead reports40,41,42,44. The ratios in between amplitudes that Gaussians were not resolved intentionally, as melanin bands are recognized to be perceptible to the chemistry environment, excitation wavelength, etc. And were thought about to it is in of interest for additional analysis. The decomposition results of the Raman spectra through high (Fl-FP = 26) and also low (Fl-FP = 3) fluorescence background space presented in Fig. 2C,D, respectively. The use of much more peaks that would take into account various other molecular contents of the skin provides the approximation unstable. Moreover, it would not lead to a an extensive increase in installation accuracy since the propose algorithm exhibited high approximation top quality with R2 = 0.960 ± 0.018 in ~ fitting selection (1,200–1,800 cm−1), i.e. Just 4% that the variance in the Raman spectra were unusual by this installation procedure.

We likewise verified the proposed installation procedure through calculating pairwise correlation coefficients that the determined Gaussian amplitudes (Fig. SI1). The high correlation in between the intensities the the Raman bands matching to lipids and also proteins I1298, I1450, I1655 to be observed (R2 ≈ 0.9), when the intensities of this bands did not correlate with the amplitudes the the melanin bands I1380 and I1570.

Besides the characteristic twin peaks situated near 1,380 and also 1,570 cm−1, the less intense vast band at about 480 cm−1 deserve to be observed in the Raman spectrum of melanin43,44. In the skin spectra, whereby amplitudes the melanin-related bands dramatically exceeded the sum of amplitudes of proteins and also lipids Raman bands, we likewise observed the band located at ≈ 480 cm−1 an ar (Fig. 2C). The presence of this band can serve as an additional argument that melanin is the source of the it was observed fluorescence and type the skin Raman signal.

Depth profiles of fluorescence and also Raman signal

We have investigated the interconnection in between the melanin concentration, as derived from the Raman spectral decomposition, and fluorescence lift intensity. The portion of melanin was calculated as the proportion of the amount of the amplitudes the the melanin-related bands to the amount of the amplitudes of all lines in the decomposition in the 1,200–1,800 cm−1 selection (Fig. 2C,D). Figure 3A demonstrates a statistically significant correlation between the calculation melanin fraction and Fl-FP worths (R2 = 0.63, p –6). However, substantial variation in the melanin fraction in spectra through high fluorescence (Fl-FP > 10) can also be noticed. We addressed this issue by examining the depth dependences of both fluorescence and also melanin Raman signals.


(A) Scatter plot of the melanin portion estimated using Raman spectral decomposition in the 1,200–1,800 cm−1 range and NIR-excited fluorescence (Fl-FP). (B) common depth profiles of the melanin fraction estimated using Raman spectral decomposition for 2 volunteers through skin kind II (blue) and III after sunlight exposure (red). (C) typical depth file of fluorescence strongness (Fl-FP) values for volunteers v skin form II (blue) and also III after sun exposure (red). Main lines represent typical values and color corridors represent standard deviation estimated using all depths profiles gained for a volunteer.

In Fig. 3B,C the representative depth profiles of the melanin portion and Fl-FP are shown, calculated because that one volunteer through skin form II and also one volunteer v skin kind III after sunlight exposure average over 6 depth-resolved file each. It deserve to be detailed that a reduced melanin fraction corresponds to reduced values the the fluorescence soot (blue curve in Fig. 3B,C). In some cases, high Fl-FP (Fl-FP ≈ 10) and melanin portion (≈ 0.4) have the right to be recognize at tiny depths (≈ 10 µm), equivalent to the stratum corneum. This to be observed for the volunteers through skin form III after sun exposure. In ~ a depth of around 35–40 µm both melanin portion and Fl-FP values demonstrated a gradual increase, if at bigger depths, the melanin portion decreased. Depth file of melanin portion and Fl-FP average within subgroups the volunteers with skin kind II and III are presented in Fig. SI2. That was found that averaging end multiple volunteers can oversmooth observed distributions early out to differences in epidermis thickness of volunteers.

The observed depth dependencies that the melanin fraction and Fl-FP worths at depths exceeding 35 µm deserve to be explained as follows. At the depths of ≈ 35–40 µm the basal class is situated where melanocytes space usually localized, hence, melanin in the basal layer is the resource of the observed local maximum the the melanin fraction and stepwise increase in Fl-FP worths at ≈ 35 µm. The diminish in the melanin fraction at bigger depths (> 40 µm) can be explained by the crossing of the dermal–epidermal junction when focusing deeper right into the skin. The stepwise behavior of fluorescence intensity suggests that at depth exceeding 40 µm the fluorophores located in the papillary dermis, e.g. Collagen and elastin, can contribute to the in its entirety fluorescence signal; therefore, the decrease of the fluorescence signal through depth is shallower 보다 the to decrease in the melanin fraction. In figure SI3, we presented the Raman spectra for the depth profiles presented in Fig. 3B,C because that the basal great (35 µm, Fig. SI3A,C) and also papillary dermis (55 µm, Fig. SI3B,D). One deserve to see the advantage of the soot of the melanin pair peaks in the 35 µm spectra, while proteins do a larger contribution come the spectra got at 55 µm.

Assessment that melanin Raman bands in the skin: non-negative procession factorization

To additional confirm the outcomes of melanin localization, we applied the non-negative matrix factorization procedure come the depth-resolved Raman spectra. Non-negative matrix factorization procedure is one unsupervised device learning technique, which allows finding representation of the input matrix through non-negative aspects as a product of 2 non-negative matrices of reduced rank51. Vectors that the output matrices have the right to be deduced as independent spectral components and also their weights, with which they add to the initial Raman spectrum. This procedure is additionally suitable because that the analysis of Raman spectra and also the recovery of Raman spectra of separation, personal, instance constituents in a complicated system52. In this work, the input matrix was composed of ≈ 2000 Raman spectra in the 800–1,800 cm−1 spectral variety with the subtracted fluorescent background bring away in same proportions for all depths. Each Raman spectrum to be preliminarily normalized come the 0–1 variety to satisfy the condition of non-negativity and also to exclude, the suspended of the observed Raman signal ~ above depth. Us made usage of six materials for decomposition, the spectra of the first four contents are displayed in Fig. 4A–D, while every the components and their depth profiles space presented in Figs. SI4, SI5.


The outcomes of the non-negative procession factorization procedure used to the Raman spectra the the skin. (AD) calculation spectral components, acquired from the administrate procedure. (E,F) common depth file of the amplitudes of decomposition contents for the volunteers with skin kind II (E) and also skin form III after sunlight exposure (F). Vertical dashed heat in (E,F) suggests the place of the dermal–epidermal junction. The spectral components and amplitude profiles are normalized by their maximum value for visibility.

The an initial two spectral contents of the non-negative matrix factorization to be notably similar to the intake Raman spectra. The first component (Fig. 4A) displayed three extreme bands at 1,298, 1,450 and also 1655 cm−1, which room characteristic because that lipids and proteins and also are current in Raman spectra v low fluorescence strongness (see e.g. Figure 2D), if the second component (Fig. 4B) was identical to the Raman spectrum that melanin (Fig. 2C).

The amplitude of ingredient 1 (blue line in Fig. 4E,F) remains virtually unchanged with depth. That is spectrum fits the Raman spectra the lipids and also proteins. The amplitude of ingredient 2, which resembles Raman spectra that melanin, exhibited comparable depth actions as the melanin portion calculated indigenous the Raman spectra decomposition (Fig. 3B). We also found out that the spectral functions of the 3rd component (Fig. 4C) are comparable to collagen (Fig. SI6), which might explain the depth dependency of the amplitude the the third component, i beg your pardon is virtually zero because that depths less than 40 µm, and exhibits rise at bigger depths (Fig. 4E,F). The observed stepwise increase of the amplitude is led to by the change from the epidermis to the dermis, wherein collagen is one of the main structural components. The observed distinctions in the place of the dermal–epidermal junction (Fig. 4E,F) could be resulted in by the differences in the epidermis thickness the volunteers.

The spectral attributes of the matrix factorization ingredient 4 (Fig. 4D) show a number of similarities v the Raman spectra the exemplary natural moisturizing element (NMF) molecules (Fig. SI7)53,54. Moreover, the obtained depth file of ingredient 4 (Fig. 4E,F) show the highest concentration in the superficial stratum corneum depths, that is specific for NMF molecules55. This facts confirm our assumption that the procession factorization ingredient 4 have the right to be attributed to the NMF molecules. However, further investigations are compelled to prove this statement.

Simple ratiometric technique for the assessment of melanin, dermal–epidermal junction and NMF circulation from the Raman spectral depth profiles

The melanin donation to the Raman spectrum in the 1,200–1,800 cm−1 range can be also estimated without any type of use that decomposition and also fitting procedures. As it have the right to be checked out in Fig. 2A,B the melanin-related tape at 1,570 cm−1 just partially superimposes through the band at 1655 cm−1. Thus, the melanin portion in the Raman spectrum can be approximated as the average intensity that Raman band in the vicinity the the 1,570 normalized come the 1655 cm−1 tape intensity that proteins. Number SI8 presents the estimation of the melanin portion as the ratio of intensities average in the 1,500–1,590 and also 1,590–1,710 cm−1 regions and also demonstrates that it is positively correlated (R2 ≈ 0.96) with the value derived from the spectrum decomposition procedure explained above.

We likewise aimed at finding some ratio that would certainly be correlated with the amplitude of the 3rd component and the 4th component derived with non-negative matrix factorization, which gives information around the dermal–epidermal junction and, presumably, NMF (Fig. 4). For this, us calculated the ratios of Raman spectra intensities for every pairs that wavenumbers in the 800–1,800 cm−1 variety and then estimated the R2 values of linear correlations in between the calculate ratios and also the amplitude the the third component acquired from non-negative matrix factorization procedure. In figure SI9A the heatmap the R2 correlation coefficients is shown. The highest possible linear correlation (R2 = 0.59) to be observed for the I1244/I1298 ratio, where intensities in ~ the indicated wavenumbers were averaged over a region ± 5 cm−1 near the center line (Fig. SI9B). That is plausible the molecular resource of such changes in Raman spectra is collagen, as it is the main component of the dermal extracellular matrix and also has an extreme C–N Raman band at 1,244 cm−153,56,57, if lipids, i m sorry are located in the epidermis, room a source of the Raman tape at 1,298 cm−150 Hence, the I1244/I1298 ratio can be a helpful tool to situate the dermal–epidermal junction indigenous the Raman spectral depth profiles. We additionally note the in ingredient 3 (Fig. 4C) additional collagen particular Raman bands at 855, 938 and 1,670 cm−1 space visible, confirming our hypothesis around its connection with collagen. This is in agreement with findings defined in58, where distinctions in the varieties 800–1,000 cm−1, 1,250–1,300 cm−1 and in the 1655 cm−1 amide bands were observed because that the dermal–epidermal junction and attributed to the Raman spectrum of collagen in the dermis.

The procedure used to discover optimal strongness ratio correlated with the amplitude the the 3rd component (“dermis”) was also applied for the decision of intensity proportion that characterizes NMF in the stratum corneum and is associated with the fourth component the non-negative matrix factorization (Fig. 4D). The ratios of the Raman spectra intensities calculation for all pairs the wavenumbers in the 800–1,800 cm−1 selection for the Raman spectra gained at depths lower than 20 µm were linearly correlated with the amplitude of component 4 the non-negative matrix factorization. The heatmap of R2 correlation coefficients is presented in Fig. SI10A. The greatest correlation (R2 = 0.83) was observed because that I1412/I1620 proportion (intensities at liked wavenumbers were average over ± 5 cm−1 spectral region, Fig. SI10B). High R2 worths were additionally observed for the ratios calculated at 886 cm−1 and 956 cm−1, 1,316 and 1,436 cm−1—the shown wavenumbers exchange mail to neighborhood maxima and minima of numerous amino acids consisted of in NMF53. In Table 2, us summarized the characteristic functions of ratiometric indicators presented in the work.

Table 2 an introduction of an easy ratiometric values used in the work, your molecular sources and depth behavior.

Spectral attributes of NIR excited fluorescence and also its relationship to melanin molecular properties

We assessed the place of the fluorescence preferably in the FP an ar by interpolation that the fluorescence background utilizing the second order polynomial function. This procedure allows determining the place of the emission maximum as the center of the parabola (Fig. 5A). We approximated the positions of the emission maxima because that the spectra with high Fl-FP worths (Fl-FP > 10, ≈ 250 spectra in total, consisting of depths native 10 to 60 µm). That was uncovered that the place of the maximum varies considerably from 1,000 cm−1 (λmax ≈ 850 nm) come 3,000 cm−1 (λmax ≈ 1,030 nm). The representative spectra with Fl-FP > 10 provided for calculation room presented in Fig. 5B and also colored according to the place of the best of the fluorescence spectra.


Assessment of spectral attributes of the NIR-excited fluorescence and also its correlation v melanin-related Raman bands. (A) Illustration the the emission maximum estimation procedure. Fluorescence background was approximated through a second order polynomial, for this reason the emissions maximum can be calculated together a center of the parabola. (B) Representative spectra v high fluorescence (Fl-FP > 10) color-coded follow to the place of the center of the parabola. (C) common Raman spectrum of the skin v high melanin content acquired for a volunteer v skin type III after sunlight exposure in ~ a depth of 38 µm (stratum basale). Melanin twin peaks space denoted as dashed lines. (D) Correlation in between the fluorescence emission maximum and also the ratio of melanin Raman bands at 1,380 and also 1,570 cm−1.

Several hypotheses that might explain the spectral properties of the it was observed fluorescence signal were considered. Firstly, it could be assumed that the emission maximum may shift due to the self-absorption effect59. Melanin exhibits one exponential long-wavelength absorption extending to the NIR range. Hence, its boosted absorption ~ above the blue leaf of the emission spectrum could lead come a much more red-shifted fluorescence. Secondly, the sports of fluorescence spectral properties can be resulted in by the heterogeneity that melanin molecule properties.

The hypothesis about the function of self-absorption in the development of fluorescence spectra band form was verified as follows. The depth dependencies the the Raman intensities videotaped in the selection of 2,800–3,000 cm−1 excited at 671 nm and in the 800–850 cm−1 selection excited in ~ 785 nm to be compared. This wavenumber areas selected because that the 671 and 785 nm excitation correspond to the emission wavelengths the 833 nm and also 839 nm respectively; thus, distinctions in attenuation the the Raman signals must be identified by the optical nature of the skin, i.e. Distinctions in scattering and absorption, related to excitation, but not to emission. We did not observe far-reaching differences between the fluorescence signal profiles gained at these two excitation wavelengths (Fig. SI11). Hence, it can be argued that absorb does no cause far-reaching changes in fluorescence band shape in vivo under the problems used in this work.

In order to understand whether molecular properties and spectral nature of melanin fluorescence are related, us analyzed the relative intensity transforms of the melanin Raman bands. For each Raman spectrum v Fl-FP > 10 the ratio of melanin tape amplitudes I1,380/I1,570 to be calculated (Fig. 5C,D) and plotted against the position of the best of the corresponding fluorescence spectrum. It was observed that the I1,380/I1,570 ratio was associated with λmax (R2 = 0.30, p –6). We confirmed that the changes in the I1,380/I1570 ratio were not an artifact of fluorescence elevator subtraction. For this we differed the fluorescence lift procedure by transforming the left selection over i beg your pardon the fluorescence background was subtracted from 500 ± 25 cm−1 to 950 ± 25 cm−1 through a 50 cm−1 step (Fig. SI12). Variations in I1,380/I1570 ratios and fluorescence emission maxima that were brought about by together fluorescence background individually were less than the variations it was observed in the experiment. Thus, it deserve to be assumed that the molecule properties of melanin manifested in the Raman spectra (evaluated as I1,380/I1,570 ratio) are additionally manifested in the spectral nature of that fluorescence (position of emission maximum). This difference in the amplitudes the melanin bands in ~ 1,380 and 1,570 cm−1 could be presumably caused by different molecular nature of melanin such as oligomers packing, amount of oxidation and also degradation, etc. And is of interest for future research.

It was additionally found that the molecular functions of fluorophores contributing to fluorescence are shown up in the spectral properties of fluorescence. Namely, the fluorescence emissions maxima established from the fluorescence background move towards lower values through depth (Fig. SI13). We suppose that this could be explained by the finding the NIR fluorescence that collagen and also elastin in the dermis (at depths > 40 µm) mainly can be blue-shifted in comparison to melanin fluorescence.

Figure 6 gift representative Raman spectra for different layers the the skin. The stratum corneum layer has been broadly studied in the literature and also distribution the lipids, water, NMF, DNA, keratin and carotenoids have actually been described in detail46,47,50,53,60,61,62,63. The maximum donation from melanin was expectably discovered near the basal layer, where the characteristic twin peaks deserve to be observed. However, depending on the skin type and exposure come sunlight, the admixture of melanin Raman bands to the Raman spectrum that stratum corneum have the right to be detected and separated using the said approaches (Figs. 2, 3, 4). Spectral decomposition actions also enabled separation between the impact of melanin and dermal ingredient of the skin, which also displayed far-ranging fluorescence upon NIR excitation that deserve to be attributed come fluorescent cross-links and also oxidation commodities in fibrillar proteins (collagens and also elastin)64,65. Hence, the defined procedures deserve to be provided to analyze Raman spectra depth profiles in the skin down to the papillary dermis.

The main idea of this occupational was come prove that melanin localization and depth distribution can it is in assessed native Raman spectra by disentangling the results of melanin and other skin constituents. The suggested method thus enables estimation that the melanin portion in various skin layers v molecular specificity that is its major difference indigenous the battery of optical methods described in Table 1. While all of that approaches are suitable for imaging that melanin in the basal layer, and also for quantification of mass melanin concentration, for the upper layers of the epidermis data translate is not that straightforward. The is, direct attributing that fluorescence in the top epidermis to melanin is no possible, as other fluorophores such together keratin66 and proteins and/or lipids oxidation products67 may likewise contribute come the emission signal.

It should additional be provided that the fluorescence intensity is prone to photobleaching, efficiently reducing the fluorescence intensity over time. The beginning of this effect is not totally identified. As the Raman bands room not topic to a photobleaching effect49, the normalization method using the ratio between fluorescence and Raman tape intensities will certainly be time-dependent and also should be considered. However, because of the brief exposure time supplied in this study, the writer propose the photobleaching dram a young role.

When concentrating on the basal layer, one deserve to use a priori expertise that melanin is the major fluorophore, and also this understanding is not applicable because that the upper layers that the skin. In contrast to this, Raman microspectroscopy-based strategy relies top top the crosscheck that the signal origin by simultaneously assessing the fluorescence and also Raman spectra in every point. Hence, the first task wherein this method can discover application is the investigation of melanin fate and its redistribution in the epidermis ~ above stress problems such together UV exposure2,68, oxidative and mechanical stress11, etc. Return this task is look at simple, the reality is that a variety of questions around the melanin fate (such together the existence of “melanin dust” in the stratum corneum13,14), kinetics and also mechanisms that its transport in normal and stress conditions and also the deterioration pathways7,11,12,13,14 require extr studies, in which the suggested technique could be useful.

Namely, after being transferred to keratinocytes with exo-/endocytosis the the melanosome main point (melanocore)69, melanin’s concentration is regulated via the asymmetric distribution of melanin between daughter keratinocytes (the one remains in the basal layer and also ‘inheriting’ many of the melanin, when the other destines to differentiate and stratify, and inherits a much smaller portion of ‘maternal’ melanin)11 and, possibly, various other mechanisms. In general, we have the right to conclude that the destruction of melanin does not occur completely and it can be found in the upper layers of the epidermis, also in the superficial areas of the stratum corneum, and also novel molecular-specific methods are required for studying the fate the melanin in the skin.

The second exciting possibility, i beg your pardon needs additional investigation, is the characterization of the melanin framework in vivo making use of its Raman spectrum tape shape. Surprisingly, regardless of its utmost significance, the optical nature of melanin are far from understood. This have the right to be portrayed by the debates around the instrument of the absorb spectrum formation in melanin, where several hypotheses room considered31,70,71. The instrument of the melanin Raman spectrum formation are no less challenging, as the heterogeneous composition of the pigment, the setup of oligomers into aggregates and also electronic interactions in ~ the aggregates should be considered42,70. The striking similarity the the melanin Raman spectrum to the of disordered carbon systems such as graphene oxide40,41,70 provides this topic even much more fascinating. Also on the superficial level of understanding, one have the right to argue the the framework of melanin have to be revealed in the Raman spectrum, e.g. In the ratio in between the twin peaks42, which was presented in this work (Figs. 5C–D). Top top the various other hand, the importance of assessing the melanin framework for melanoma characterization to be well demonstrated in the works of Warren et al.24,28,29,30,31, whereby the oligomers stacking setting was shown to be shown up in the transient absorption properties. Based on the researches of synthetic melanins42, we believe that additional information about the melanin company in vivo have the right to be obtained from its Raman spectrum and, possibly, fluorescence spectrum, which displayed surprisingly large variation the the band shape (the position of maximum varied from 860 to 1,000 nm upon 785 nm excitation) in our experiment (Fig. 5B). In addition to the possibility to precisely localize melanin in the skin, this can open up new diagnostic ways of predicting and analyzing melanin-related disorders.

In this work, the distribution of melanin in the epidermis to be assessed using confocal Raman microspectroscopy by characteristic wide Raman “twin peaks” the melanin centered at ≈ 1,380 and ≈ 1,570 cm−1 as well as by NIR excited fluorescence the melanin. The argued Raman-based approach allows estimating the melanin portion in different layers of the skin through molecular specificity through disentangling the Raman spectrum that melanin in the 1,200–1,800 cm−1 variety with the Raman signal of proteins, lipids and also other constituents. The was uncovered that lot of approaches, namely, installation of Raman spectra by many Gaussian lines, unsupervised non-negative matrix factorization, as well as basic ratiometric indices have the right to be successfully applied to gain insights into the melanin circulation in the epidermis, consisting of the stratum corneum. The administer procedure also detailed information about the ar of the dermal–epidermal junction and distribution of NMF.

The depth file of NIR excited fluorescence were uncovered to correlate well through the melanin fraction as established from the Raman spectra in the epidermis. However, high NIR fluorescence was likewise observed in the dermis, suggesting that it could be source from other skin contents such as oxidatively modified proteins.

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We have additionally assessed the spectral band form of the melanin-related NIR fluorescence. That was uncovered that the place of the fluorescence emission maximum correlates with the ratio of the amplitudes the the melanin bands centered at ≈ 1,380 and ≈ 1,570 cm−1. Therefore, we believe that not only information about the distribution of melanin, but additionally insights into its molecule organization have the right to be assessed by the combined Raman and NIR-fluorescence approach, which, in turn, can administer a new understanding that the habits of melanin in healthy and also pathological skin.