Absolute luminescence quantum yield for nanosized carbon particles in water as a function of excitation wavelength

Journal Pre-proof Absolute luminescence quantum yield for nanosized carbon particles in water as a function of excitation wavelength

Kirill A. Laptinskiy, Sergey A. Burikov, Svetlana V. Patsaeva, Igor I. Vlasov, Olga A. Shenderova, Tatiana A. Dolenko PII:

S1386-1425(19)31269-7

DOI:

https://doi.org/10.1016/j.saa.2019.117879

Reference:

SAA 117879

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date:

6 August 2019

Revised date:

18 November 2019

Accepted date:

29 November 2019

Please cite this article as: K.A. Laptinskiy, S.A. Burikov, S.V. Patsaeva, et al., Absolute luminescence quantum yield for nanosized carbon particles in water as a function of excitation wavelength, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2019), https://doi.org/10.1016/j.saa.2019.117879

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© 2019 Published by Elsevier.

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Absolute luminescence quantum yield for nanosized carbon particles in water as a function of excitation wavelength Kirill A. Laptinskiy*a,b, Sergey A. Burikova, Svetlana V. Patsaevaa, Igor I. Vlasovc, Olga A. Shenderovad, Tatiana A. Dolenkoa a

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Corresponding author e-mail: [email protected]

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Physical Department M.V. Lomonosov Moscow State University, Moscow, Leninskie Gory 1/2, 119991 Russia b Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Leninskie Gory 1/2, Moscow, Russia c A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, 119991 Moscow, Russia d Adamas Nanotechnologies, Inc., 8100 Brownleigh Dr, Suit 120, Raleigh, NC 27617, USA

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Keywords: luminescence quantum yield, excitation wavelength, carbon nanoparticles, detonation nanodiamonds, carbon dots.

1. Introduction

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Abstract: The absolute luminescence quantum yield Q as a function of excitation wavelength λex in a wide spectral range 270-470 nm was measured for the first time for the group of carbon nanoparticles dispersed in water: carbon dots (CD), detonation nanodiamonds (DND), as well as detonation nanodiamonds decorated with carbon dots (CD-DND). The luminescence quantum yield for DND increased after functionalization; the CD-decorated DND demonstrated significantly higher Q values in the UV region of excitation. We found that the quantum yield for CD luminescence is 4-8 times higher than that for CD-DND luminescence, and 20 times higher than that for DND luminescence. Roughly three spectral regions can be distinguished within the Q(λex): below 330 nm, 330-390 nm and 390470 nm. Conclusions are drawn about the number of chromophores of the studied nanoparticles and transfer of photoexcitation energy in the systems under consideration.

Carbon nanoparticles (CNP) belong to variegated class of materials with different chemical structures created through optimization of synthesis, surface modification, doping, and purification conditions. CNP have been successfully employed in biomedical applications due to the combination of such desirable properties as stable photoluminescence (compared to traditional luminescent markers), possibility of postsynthesis modification of their surface, high dispersion ability in water and low toxicity [1-5]. The developed surface chemistry of CNP makes it easy to load them with pharmaceutics and promote them as theranostic nanoagents. One of the main challenges of biomedicine is creation of multifunctional nanoagent which combines the properties of drug-carrier and fluorescent biomarker. When developing such a nanoagent, it is necessary to study thoroughly its luminescent properties. This concerns both clarification of mechanisms of luminescence formation and influence of various factors on its emission (temperature, interaction with environment molecules, etc.) [6,7].

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From the view of prospects of choosing one or the other material as a fluorescent marker, one of the key parameters is the luminescence quantum yield (Q) which is defined as the ratio of the number of emitted to absorbed photons. Measurements of the luminescence quantum yield of carbon quantum dots (CD) were carried out by many authors; it is stated that absolute Q values range from units to tens of percent [9-11]. Such a large spread is obviously caused by the fact that carbon dots are produced from various raw materials and according to different technologies. However, at present there are not so many studies devoted to the experimental estimation of Q for detonation nanodiamonds (DND) [8]. Apparently, this is caused by the experimental difficulties encountered in solving this problem, availability of the large number of different manufacturers of DND. In general, the question of standardization of DND characteristics remains open; and the Q values may be utilized for their characterization. Even more intriguing is to measure luminescence quantum yield as a function of excitation wavelength, Q(λex), for nanosized particles. One can find few studies devoted to investigation of the dependence of luminescence quantum yield of CNP on the excitation wavelength λex [12,13]. If the Q depends strongly on λex this feature can be used to manipulate the emission spectrum of CNP through the proper choice of excitation wavelength/source.

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A number of methods were described to measure experimentally the absolute values of luminescence quantum yield [14]. Currently, the method of reference substance [15-18] is used mostly often to determine Q for dyes and natural luminophores in solutions. This method is based on the comparison of the luminescence intensity of the solution of reference substance with known value of luminescence quantum yield and the intensity of the studied luminescing substance. The formula for Q calculation includes also the absorbances at the wavelength of excitation for both reference solution and solution under investigation. The large set of reference dyes was suggested in literature which value of luminescence quantum yield does not depend on the excitation wavelength in a wide spectral range and is known with sufficient accuracy [19].

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However, a number of difficulties complicates the implementation of this method for measuring the Q values for nanoparticles in suspensions and in turbid media. One of them is the correct calculation of absorbance in turbid solutions and suspensions with high scattering intensity. When registering the optical density of the suspension of nanoparticles using a spectrophotometer both absorption and scattering contribute to measured extinction. To eliminate errors associated with the effect of scattering, several approaches were proposed. Thus, the paper [20] describes application of a reference substance with scattering properties similar to the object under study. This is achieved by addition to the reference sample the non-luminescent scattering particles of the same size that are present in the test sample. This approach allows to take into account the contribution of scattering in the measurement of extinction and thus to facilitate more correct determination of the quantum yield of luminescence. However, in practice it is difficult to find a reference dye with scattering particles of the desired size for each case. Another approach to solve this problem can be implemented in the cases when the extinction of the testing sample is mainly caused by scattering. If the shape of the wavelength-dependence of the scattering intensity is known, it can be subtracted from the real extinction spectrum and one can obtain the value of the "true" absorption spectrum without scattering. The problem of interpretation of extinction spectra of aqueous suspensions of nanodiamonds was studied in detail [21-25]. The extinction spectrum of nanodiamonds (with sizes from units and tens of nm) dispersed in water in the UV and blue spectral regions is determined primarily by elastic scattering on

Journal Pre-proof small particles, the intensity of which is proportional to -4 (where  is the wavelength of radiation) [25]. Deviations from this law are determined by the degree of surface cleaning of nanodiamonds and their size. According to the authors [22], the absorption of DND is mainly caused by the layer of carbon with sp2-hybridization on the surface of the nanodiamond.

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This study presents a comparative analysis of the values of the absolute luminescence quantum yield as a function of excitation wavelength in a wide spectral range 270-470 nm was measured for the first time for the group of carbon nanoparticles dispersed in water: carbon dots (CD), detonation nanodiamonds (DND), as well as detonation nanodiamonds decorated with carbon dots (CD-DND). For more correct determination of Q values for CNP in aqueous suspensions, contribution caused by the scattering of light on small nanosized particles (with the size much smaller than the wavelength) was subtracted from the registered extinction spectra. On the basis of the obtained dependencies Q(λex) the conclusions are drawn about the heterogeneity of chromophores in investigated nanoparticles.

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2. Materials and Methods 2.1. Preparation and Characterization of CNP Suspensions in Water

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Aqueous suspensions of the following CNP samples provided by the Adamas Nanotechnologies were used as research objects:

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1). Nanodiamonds of detonation synthesis (DND). The synthesis technique is described in detail in Ref. [26]. The DND surface was characterized by means of IR absorption spectroscopy (Varian 640-IR FT-ATR IR spectrometer with module with ZnSe crystal was used). It was found that carboxyl groups dominate among the surface groups. The value of ζ-potential of DND aqueous suspension was −40 ± 2 mV.

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2). Nanodiamonds decorated with carbon quantum dots (CD-DND) [27]. Samples were obtained by exploding an oxygen-deficient explosive mixture of trinitrotoluene with hexogen (50:50 wt%) in a closed steel chamber using CO2 cooling media. The synthesis technique is described in detail in Ref. [28]. The value of ζ- potential of CD-DND aqueous suspension was −35 mV.

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3). Carbon dots obtained by oxidation of micro-and nanographite with a mixture of sulfuric and nitric acids (3:1 95-98% sulfuric acid to 68% nitric acid) at a continuously maintained temperature of 128°C for 2 hours. The technique is described in detail in Ref. [29]. The value of ζ- potential of CD aqueous suspension was −22.5 mV.

Ultrapure deionized water obtained by Millipore Simplicity UV water purification system was used to prepare the following suspensions of nanoparticles: DND with concentration of 0.1 mg/ml (pH = 6.39), CD-DND with concentration of 0.025 mg/ml (pH = 6.50), CD with concentration of 0.01 mg/ml (pH = 7.71). The nanoparticle sizes in prepared aqueous suspensions determined by dynamic light scattering on the analyzer ALV-CGS-5000/6010 (Langen, Germany) were found as the following: 5 ± 1 nm (DND), 6 ± 1 nm (CD), and 8 ± 1 nm (CD-DND). In the study of aqueous suspensions of carbon nanoparticles, especially nanodiamonds, the possible aggregation of particles in suspensions is an important problem. As it was established by many

Journal Pre-proof researchers, detonation nanodiamonds can form aggregates in aqueous suspensions [30]. To date, a large number of cleaning and surface modification methods have been developed that prevent the formation of aggregates in suspensions [31–33], however, the issue of stability of aqueous suspensions remains important. Under certain circumstances, carbon dots can also form aggregates, and the presence of aggregates can lead to a change in the luminescent properties of particles [34].

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2.2. Optical Spectroscopy of CNP Aqueous Suspensions

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In our work, the stability of the studied dispersions was verified by repeated measurements of the particle size and their ζ potential, which were carried out for 3 months. It was found that these parameters did not change over time, which, together with high absolute values of the ζ potential, suggests a high colloidal stability of the suspensions. In addition, the luminescence spectra were recorded at constant controlled parameters of the water environment, such as temperature, pH value, a change in which could cause aggregation of nanoparticles.

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Absorption spectra of aqueous suspensions of CNP were obtained using the Solar RV2201 spectrophotometer equipped with a pulsed xenon lamp as a light source. The spectra of the optical density of CNP suspensions were recorded in the wavelength range from 200 to 750 nm with increments of 1 nm in quartz cuvette with an optical path length of 1.00 cm. The concentration of suspensions was selected so that the optical density in the visible region of the spectrum did not exceed 0.1 to avoid the inner filter effect in luminescence measurements. In order to separate the contribution of absorption to the extinction spectrum, the wavelength-dependent baseline described as λ-4 (the scattering on the particles with the sizes less than the wavelength of the radiation) was subtracted from all registered spectra. The difference between the recorded spectrum and this scattering approximation allowed to distinguish the contribution caused by light absorption by substances under investigation [23].

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Luminescence spectra of nanoparticles in aqueous suspensions were recorded using luminescence spectrometer Solar CM 2203 in the range of excitation wavelengths from 270 nm to 480 nm with increment of 5 nm and registration wavelength up to 750 nm with increment of 1 nm. Examples of luminescence spectra for DND aqueous suspensions are presented only for the spectral region starting from 330 nm due to the strong influence of scattering in the shortwave UV region. Suspensions were placed in standard quartz cuvettes for fluorimetry, registration was carried out in 90-degree geometry. The measured luminescence intensity in each spectral channel was corrected for absorption at the excitation and registration wavelengths using the Equation 1:

𝐼𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝐼𝑟𝑒𝑔𝑖𝑠𝑡𝑒𝑟𝑒𝑑 · 100.5·(𝑂𝐷(𝜆)+𝑂𝐷(𝜆𝑒𝑥 )) ,

(1)

where Icorrected is the luminescence intensity at the emission wavelength λ after correction, Iregistered is the registered luminescence intensity at the emission wavelength λ, OD(λ) is the optical density at the emission wavelength of λ, λex is the excitation wavelength. To calculate luminescence quantum yield we use corrected luminescence intensities integrated over the emission wavelength range for each excitation wavelength.

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Further data processing involved subtraction the contribution of water Raman scattering and extrapolating the luminescence signal to the excitation wavelength (Figure 1), more detailed processing algorithm is described in the Supplementary materials. We estimated the inaccuracy in the wavelength-integrated luminescence intensity as not exceeding 3%. This inaccuracy was taken into account in the calculation of the error in determining the Q values for our samples.

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Figure 1. Contribution of CNP luminescence in registered emission spectrum in suspension.

2.3. Determination of the luminescence quantum yield of nanoparticles

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Luminescence quantum yield is defined as the ratio of the number of emitted photons to that of absorbed photons. Determination of the CNP luminescence quantum yield in suspensions was carried out using the reference dye method [35] according to the (Equation 2):

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𝑄 = 𝑄𝑟

𝑂𝐷𝑟 𝐼 𝑛 2 ( ) , 𝑂𝐷 𝐼𝑟 𝑛𝑟

(2)

where Q - the luminescence quantum yield of nanoparticles, OD is the optical density at the wavelength of excitation λex, I is the wavelength-integrated luminescence intensity, n is the medium refractive index. Index r means the use of known parameters of the reference dye (Rhodamine 6G solution in ethanol). In order to test our results, the spectra of the ethanol solution of Rhodamine B were also measured and the quantum yield of its fluorescence was determined, which turned out to be 0.66, which is in a good agreement with measurements by other researchers [36].

3. Results and Discussion 3.1. Spectrophotometry In Figure 2 the extinction spectrum in the spectral region 200-750 nm, the baseline ~ λ-4 caused by light scattering on nanoparticles, and the absorption spectrum after subtraction of the baseline are presented.

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Figure 2. Extinction spectra and calculated absorption spectra of the CNP aqueous suspensions.

Journal Pre-proof From the obtained data it can be seen that the extinction spectra of the DND and CD-DND have no obvious features and represent monotonically decreasing curves with decreasing absorbances along with wavelength rising. It should be noted that in the spectra of the optical density of the DND and CD-DND, in contrast to CD, the scattering clearly dominates. In the CD absorption spectrum, the features are apparently observed around 232, 268, 302 and 367 nm, which corresponds to the transitions π-π*, π-π*, nπ*, n-π*, respectively [37,38].

3.2. Luminescence spectra

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In Figure 3 the processed luminescence spectra of CNP aqueous suspensions are presented as 2D plots of excitation/emission matrixes (EEM); EEM spectra were normalized on the maximum intensity on those plots.

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Figure 3. Luminescence spectra of DND (a), CD-DND (b), CD (C) at different excitation wavelengths.

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Luminescence of aqueous suspensions of the investigated CNP represents a wide band, occupying almost the entire visible range. As it can be seen in Figure 3 (middle), the position of the emission maximum for CD-DND water suspension depends significantly on the excitation wavelength. When the excitation wavelength rises from 270 to 470 nm, the bandshape of the luminescence emission spectrum changes, which may manifest the presence of a set of various chromophores, characterized by their own absorption and luminescence quantum yield. Similar, but more pronounced transformations in the luminescence spectra were observed for the CD aqueous suspension. With an increase in the excitation wavelength from 270 to 470 nm in addition to a significant red-shift of luminescence maximum, there is seen a significant change of the shape of the emission band: a feature in the luminescence spectrum observed 430 nm decreases with rising λex and completely disappears at λex =365 nm. Such behavior of the CD luminescence spectra also indicates spectral heterogeneity, the presence of several types of chromophores.

3.3. The Luminescence Quantum Yield for CNP in Aqueous Suspensions In Figure 4 the dependences of the obtained values of luminescence quantum yields for the studied samples on the excitation wavelength are presented.

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Figure 4. Dependences of the luminescence quantum yield of the CNP samples in water on the excitation wavelength. The error bar is shown as an example at one of the excitation wavelengths.

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As it follows from the obtained data, the quantum yield of CD luminescence is 4-8 times higher than the quantum yield of CD-DND luminescence, and 20 times higher than that of DND luminescence. For example, the luminescence quantum yield at the wavelength of excitation 330 nm for DND, CD-DND, and CD are equal to 0.21%, 0.67% and 4.02%, respectively. Thus, the luminescence quantum yield for DND increased after functionalization; the CD-decorated DND demonstrated significantly higher Q values in the UV region of excitation.

3.4. Discussion

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As it can be seen from the presented results (Figure 4), the luminescence quantum yield of the of the investigated CNP aqueous suspensions depends on the excitation wavelength and differ significantly from each other. In the Q(λ)-dependence for the DND suspension in water, two spectral regions can be distinguished, 330-390 nm and 390-470 nm, where the values of luminescence quantum yields are constant within the error (Figure 4, a). This situation may occur in presence of two types of chromophores in the sample. The presence of two different fluorophores with luminescence lifetimes 1.4 ns and 6.2 ns for the carboxylated DND was shown [39]. It was found that the largest contribution to luminescence, more than 90%, makes "fluorophore" with shorter luminescence lifetime.

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The dependence of the Q(λex) quantum yield of luminescence of aqueous CD suspension on excitation wavelength has a complex structure (Figure 4, с), indicating the presence of several different chromophores. The peculiarity of the excitation-dependence of the luminescence quantum yield (the minimum around 365 nm) coincides with the previously noted feature in the absorption spectra. Apparently, one of the CD chromophores absorbs the radiation with the wavelength at 365 nm, but does not emit luminescence.

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In the dependence Q(λex) for CD-DND suspension in water one can observe monotonic decrease of the luminescence quantum yield along with increasing excitation wavelength (Figure 4, b). This dependence can be a consequence of the fact that in this case there is an overall influence of chromophores of DND and CD on the luminescence spectrum. These chromophores interact in a complex way with each other: the chromophore groups of CD covering the surface of DND and the chromophore groups of DND themselves can transfer the energy of photoexcitation to the emitting centers with partial energy loss. It causes the large Stokes shift (the difference between the excitation wavelength and the emission wavelength greater 100 nm). There can be variety of such combinations of chromophore – emitting center. The diversity of these pairs is affected by processes of interactions of nanoparticles surface groups with the surrounding water molecules. It is well known that these interactions significantly affect the luminescence of nanodiamonds and CD [8,40]. Thus, the obtained dependence of the quantum yields of CD-DND luminescence in water on the excitation wavelength indicates the existence of the continuum of chromophores in these nanoparticles and significant degradation of the photoexcitation energy in the process of its transfer.

Conclusions

Journal Pre-proof For the first time the values of quantum yield of luminescence of DND, CD and CD-DND suspended in water were determined in a wide range of excitation wavelengths. As it follows from the obtained data, the quantum yield for CD luminescence is 4-8 times higher than that for CD-DND luminescence and 20 times higher than that for DND luminescence. The luminescence quantum yield for DND increased after functionalization; the CD-decorated DND demonstrated significantly higher Q values in the UV region of excitation. For the first time significant dependence of the values of luminescence quantum yield on the excitation wavelength was found for CNP in a wide range of excitation wavelengths. This finding can open the perspective to manipulate the emission spectrum of CNP through the proper choice of excitation wavelength/source.

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The obtained results support the hypothesis of the presence of several types of luminophores in CNP, interacting in a complex way with each other and with solvent molecules. This explains the complex nature of the dependence of luminescence quantum yield on the wavelength of excitation. The Q(λex) dependences may be used for characterization of different nanoparticles.

Acknowledgements

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The work was supported by the grant of RSF №17-12-01481.

Conflict of Interest

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The authors declare no conflict of interest.

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CRediT author statement:

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Kirill A. Laptinskiy: Methodology, Investigation, Writing - Original Draft, Writing - Review & Editing, Sergey A. Burikov: Investigation, Writing - Original Draft, Writing - Review & Editing, Svetlana V. Patsaeva: Methodology, Writing - Review & Editing. Igor I. Vlasov: Resources, Olga A. Shenderova: Resources, Tatiana A. Dolenko: Supervision, Writing - Original Draft, Writing - Review & Editing

Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical abstract

Journal Pre-proof Highlights:

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Luminescence quantum yield of carbon nanoparticles depends on excitation wavelength The luminescence quantum yield of detonation nanodiamond is around 0.2% The luminescence quantum yield of nanodiamond covered with carbon dots is around 1%

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