Near-Infrared Absorbing Solid Lipid Nanoparticles Encapsulating

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C: Physical Processes in Nanomaterials and Nanostructures

Near-Infrared Absorbing Solid Lipid Nanoparticles Encapsulating Plasmonic Copper Sulfide Nanocrystals Fabio Vischio, Elisabetta Fanizza, Vito De Bellis, Teresa Sibillano, Chiara Ingrosso, Cinzia Giannini, Valentino Laquintana, Nunzio Denora, Angela Agostiano, Marinella Striccoli, Maria Lucia Curri, and Nicoletta Depalo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05897 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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The Journal of Physical Chemistry

Near-Infrared Absorbing Solid Lipid Nanoparticles Encapsulating

Plasmonic

Copper

Sulfide

Nanocrystals

Fabio Vischio, a,b, ‡ Elisabetta Fanizza

a,b,*, ‡

Vito De Bellis,a, † Teresa Sibillano,c Chiara

Ingrosso,b Cinzia Giannini,c Valentino Laquintana,d Nunzio Denora,b,d Angela Agostiano,a,b Marinella Striccoli,b M. Lucia Curri,a, b Nicoletta Depalob, *

a Università

degli Studi di Bari Aldo Moro, Dipartimento di Chimica, Via Orabona 4,

70125-Bari, Italy

b CNR-Istituto

per i Processi Chimico-Fisici SS Bari, Via Orabona 4, 70125 - Bari, Italy

c CNR-Istituto

di Cristallografia, Via Amendola, 122/O, 70126 Bari, Italy

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

d Università

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degli Studi di Bari Aldo Moro, Dipartimento di Farmacia – Scienze del

Farmaco, Via Orabona 4, 70125 - Bari, Italy.

Abstract A novel NIR absorbing nanoformulation is fabricated by efficiently encapsulating plasmonic Cu2xS

nanocrystals (NCs) within the core of low-density lipoprotein (LDL)-mimetic solid lipid

nanoparticles (SLN). Cu2-xS NCs are ideal candidates as photothermal or imaging agents for in vivo cancer treatment thanks to their plasmonic properties, low toxicity, bio-degradability and low cost. Their incorporation in the LDL-mimetic nanocarriers enhances the nanoformulation potential in biomedical applications resulting in nanosystems able to provide the concomitant delivery of anticancer molecules and cancer diagnosis and/or therapy. Beside the comprehensive characterization of the prepared nanostructures, the paper aims to tackle the accurate determination of the NC concentration in the final nanoformulation. Since both photothermal therapy and imaging efficiency strongly depend on concentration of the nanoagents, the availability of a real time and nondestructive approach for the determination of NCs concentration in the SLNs is essential for clinical studies addressing the possible administration of the nanoformulation for in vivo applications. Here Mie theory and Drude model based fitting procedure of the experimental results from the morphological and spectroscopic characterization, is proposed for the definition of the plasmonic properties of nanoformulation and the determination of the concentration in terms of Cu2-xS NCs and SLNs. Introduction


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Among the plasmonic nanoparticles (NPs), such as metal carbon or transition metal sulfide/oxide based NPs and organic nanomaterials,1 that possess extraordinary properties, resulting able to convert a bio-transparent electromagnetic radiation into heat, copper sulfide Cu2-xS nanocrystals (Cu2-xS NCs) have been proposed as novel nanoheaters for plasmonic photo-thermal therapy (PPTT).2 Indeed, Cu2-xS is a self-doped p-type metal chalcogenide semiconductor which exhibits tuneable localized surface plasmon resonance (LSPR) in the near-infrared (NIR) spectral range. In such a selfdoped semiconductor, the cation deficiency in the structures produces free positive charge carriers that undergo collective oscillations when excited with an appropriate electromagnetic field.3 Conversely to what found for conventional metal nanostructures, the plasmonic properties of Cu2-xS NCs in the near and mid IR region do not require any specific shape engineering, since they are relevant already in Cu2-xS quantum dots of few nanometers in size. In addition, the plasmon resonance can be easily modulated even during the synthesis or in a post synthesis redox reaction, by simply regulating copper deficiency and density of hole carriers. Then, Cu2-xS NCs represent ideal candidates as potential PPTT agents for the cancer treatment, thanks to their peculiar plasmonic


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properties and low toxicity, as well as their bio-compatibility and the lower cost compared to noble metal NPs.4 Furthermore, Cu2-xS NCs can also function as effective contrast agents in the enhanced magnetic resonance imaging (MRI) or infrared thermal imaging or photoacoustic imaging for multimodal imaging guided PPTT.5,

6

Interestingly, the

incorporation of Cu2-xS NCs in polymeric or lipid-based nanocarriers results in original nanoformulations enabling, concomitantly, the delivery of anticancer molecules and cancer therapy based on multimodal imaging guided PPTT and chemotherapy.

Here, a new nanoformulation based on low-density lipoprotein (LDL)-mimetic solid lipid nanoparticles (SLNs) loaded with Cu2-xS NCs has been obtained (N-SLNs). Among the different nanocarriers,7-

13

SLNs have been selected as good candidates for the

encapsulation of Cu2-xS NCs in order to achieve nanoformulations potentially useful for optically induced heating and controlled drug release. Moreover, SLNs, mainly formed by crystalline and physiological fatty acids, are generally recognized as safe and are characterized by high storage stability against degradation, aggregation and coalescence.14,15

SLNs have been extensively proposed as drug/gene delivery


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nanosystems as they present several advantages, including low toxicity due to biodegradable nature of lipid vector,16 high bio-absorbability,14 ease of preparation, scalability of production and sterilization,15 and high loading efficiency of lipophilic drugs within their internal core or of negatively charged nucleic acids on their large surface.14 In addition, SLNs possess good ability to prevent drug degradation and modulate drug release.15, 16 So far, only few reports on the encapsulation of inorganic NPs in the lipid core of SLNs have been presented.

17- 20

For example, fluorescent CdSe/ZnS NPs were

encapsulated in SLNs formed of a lipid matrix of stearic acid and mono-, di- e triglycerides of palmitic acid,

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Au nanospheres were incorporated in the matrix of SLN

composed of dodecanoic acid and Tween 20 14 or, also, superparamagnetic -Fe2O3 NPs were embedded in trilaurin

21

or cetyl palmitate

17

based SLNs. Here, surface-modified

cationic SLNs composed of natural protein-free components of low-density lipoproteins (LDL) have been employed to obtain LDL-mimetic nanovectors, optically transparent in the NIR region, where the plasmonic properties of Cu2-xS NCs are conveyed, resulting in a nanoformulation potentially useful for theranostic purposes.


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Since PPTT or imaging efficiency strongly depends on laser power irradiation and concentration of the nano-agents, the accurate determination of the Cu2-xS NCs concentration in the SLNs become essential to evaluate the maximum dose of administrated nanoformulations into the systemic circulation, fundamental for in vivo diagnostic and therapeutic applications.22 Furthermore, the concentration of the nanoformulation is strictly required in order to thoroughly characterize the product for any regulatory approval.23 Moreover, pharmacopoeial requirements, concerning the safety of pharmaceutical dosage forms, limit the nominal content of the particle number per volume for injectable pharmaceutical suspensions based on particulate matter. 24

An accurate determination of the concentration by using a nondestructive method is a hard challenge. In fact, while parameters such as particle size and size distribution or physical and chemical properties can be confidently obtained by conventional analytical tools, there are currently no certified and widely applicable experimental methods for measuring number concentration of NPs in the diameter range from roughly 1 to 100 nm. 22, 25

Here, a prompt, easily accessible and nondestructive approach has been developed


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and proposed for the determination of N-SLNs and Cu2-xS NCs concentration in the nanoformulation, for the future evaluation of dose-response relationship of the novel designed nanoformulation. The method combines experimental data, obtained by optical and morphological techniques, namely UV-Vis-NIR absorption spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM), with theoretical information derived from Maxwell equations, solved according to Mie theory and Drude model for plasmonic NCs. Indeed, this approach offers a valuable tool for the reliable determination of the technical specifications of the proposed nanoformulation, required for any regulatory approval, and for their future use in the clinical application as effective agents for PPTT or imaging and drug delivery systems.

Experimental section

Materials: copper (I) chloride, CuCl, oleylamine (OLAM, technical grade 70%), sulphur powder, oleic acid (OA, 90%), 1-octadecene (ODE, 90%), cholesteryl oleate, glyceryl trioleate, cholesterol and phosphotungstic acid (99.995%) were purchased from SigmaAldrich (St. Louis, MO). 1,2-di-(9Z-octadecenoyl)-sn-glycero-3-phosphoethanolamine


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(DOPE), 3-[N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol hydrochloride (DCchol), and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy (polyethylene glycol)-2000] (ammonium salt) (DSPE-PEG, Mw = 2 kDa) were obtained from Avanti Polar Lipids (Alabaster, AL). All other chemicals and reagents were of analytical grade. A Milli-Q gradient A-10 system (Millipore, 18.2 MΩ cm, organic carbon content≥4μg/L) was used to obtain water for the preparation of all employed aqueous solutions.

Synthesis of Cu2-x S NCs: The synthesis of Cu2-xS NCs was performed according to literature reported protocols with some modifications in the synthetic and purification steps.26-28 The strategy relies on the hot injection method, under standard airless conditions. The precursor solutions were prepared under free air conditions using a glove box. In a three-neck flask, 1 mmol of CuCl was dissolved into 15 mL of ODE, 3 mL (9 mmol) of OA and 7 mL (~20 mmol) of OLAM. In another three-neck flask, 1 mmol of sulphur were dissolved into 2.5 mL of ODE and 2.5 mL (8 mmol) of OA. Both mixtures were heated under vigorous magnetic stirring at 100°C under vacuum for 30 minutes and subsequently the temperature was increased up to 180°C under N2 flow for 15 min to


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allow the complete decomposition of the precursors. At this step, the copper solution gradually turned from deep blue to light yellow and finally to deep brown and the sulphur precursor solution turned to reddish. The sulphur precursor was quickly cooled down at room temperature and then swiftly injected at 180°C into the flask containing the copper precursor. The mixture suddenly turned to dark brown and was left stirring at 180°C for 15 minutes. The reaction mixture was cooled down to room temperature and the as-

synthesized NCs were purified by repeated cycles of precipitation with ethanol and centrifugation. Finally, the NCs were dispersed in tetrachloroethylene (TCE) to yield stable and optically clear colloidal solution, for further characterization.

Preparation of solid lipid nanoparticles loaded with Cu2-xS NCs: Plasmonic Cu2-x S NCs dispersed in TCE were treated with ethanol, recovered by centrifugation and dispersed in chloroform (17.2 g/L). SLNs loaded with the NCs (denoted as N-SLN) were prepared by hot homogenization-evaporation technique according Bae et al.

18

with optimizations.

Cholesteryl oleate (20.5 mg), glyceryl trioleate (1.5 mg), DOPE (7 mg), cholesterol (5 mg), DC-chol (14 mg), DSPE-PEG (0.05 mg), and Cu2-x S NCs were dissolved in chloroform


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(1 mL). For comparison, empty SLNs (denoted as E-SLNs) were prepared by using the same lipid mixture and procedure without addition of Cu2-x S NCs. The organic solution was mixed with pure water (5 mL), and then sonicated for 5 minutes by using a Branson Sonifier 150. The resulting oil-in-water emulsion was transferred into a round-bottom flask, and then the organic solvent was rapidly evaporated under reduced pressure at 60°C by using a rotary evaporator (Buchi, Switzerland). Centrifugation (5000xg for 1 min) was carried out to remove any excess of plasmonic NCs, possibly not embedded in the lipid core of SLNs. Finally, SLNs were further purified by using Centriplus YM100 (Millipore, Bedford, MA) with water (Mw cutoff = 100 kDa) and then stored at 4°C until use. Empty E-SLNs were achieved following the same protocols previously described for the preparation of N-SLNs, but just without adding the NCs.

Transmission electron microscopy investigation: The size and morphology of Cu2-x S NCs and SLNs were investigated by Jeol JEM 1011 transmission electronic microscope (TEM) operating at 100 kV and equipped with a W electron source anda CCD high resolution camera. Deposition of the “as synthesized” plasmonic NCs was carried out by dipping a


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carbon coated copper grid in TCE dispersion of Cu2-x S NCs and by letting the solvent to evaporate. The particle size distribution (PSD) of Cu2-x S NCs samples was obtained by counting at least 150 particles for each sample by means of a freeware Zeiss AxioVision analysis program. In particular, the average NCs and SLNs size was measured and the percentage relative standard deviation (%) was calculated in order to define their size distributions.

For the negative staining TEM observation of SLNs, the sample deposition was performed by dropping 5 µL of aqueous SLNs dispersion on the grid. After complete solvent evaporation, 5 µL of a 2% (w/v) phosphotungstic acid solution were cast on the grid and left for 30 seconds. Staining agent excess was removed by rinsing the grid with ultrapure water and blotting, at the edge of the grid, with filter paper. The sample on the grid was left to dry overnight and finally stored in a vacuum chamber until analysis.

Spectroscopic analysis: Absorption measurements were performed by means of a UVVis-NIR Cary 5 spectrophotometer (Varian). The optical measurements on the Cu2-x S NCs dispersion were carried out at room temperature on their dispersion obtained directly


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from synthesis after purification. SLNs were investigated in ultrapure water at room temperature.

Particle size, size distribution and surface charge evaluation: A Zetasizer Nano ZS, Malvern Instruments Ltd., Worcestershire, UK (DTS 5.00) was used to evaluate hydrodynamic diameters and colloidal stability of the SLNs. In particular, average size, size and volume distributions were determined after sample dilution in demineralized water. Size distribution is described in terms of polydispersity index (PDI). The ζ-potential measurements, i.e. the surface charges, were carried out by using a Laser Doppler Velocimetry (LDV) after sample dilution in KCl aqueous solution (1 mM). All reported data are presented as mean values ± standard deviation of three replicates.

Determination of SLN yield and NCs encapsulation efficiency: A lyophilized aliquot of suspensions of synthetized SLNs was weighted and the determination of effective amount of SLN wSLN was obtained using the following equation:

wSLN 

VSLN waliq Valiq


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where VSLN is full volume of formulation and Valiq , waliq are volume and weight of lyophilized aliquot, respectively. Afterwards, the solid was dissolved in warm ethanol at 40°C and was centrifuged (5000xg for 1 min) in order to separate Cu2-x S NCs from lipids. The pellet was washed with ethanol and centrifuged three times and finally left to dry in a vacuum chamber. The obtained powder, which represents the effective amount of Cu2-x S NCs encapsulated in the lipid core of N-SLNs, was weighted and encapsulation efficiency % EECu

2 x S

was calculated according to the following formula:

%𝐸𝐸𝐶𝑢2 ― 𝑥𝑆 =

𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑒𝑛𝑐𝑎𝑝𝑠𝑢𝑙𝑎𝑡𝑒𝑑 𝑁𝐶𝑠 ∙ 100 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑁𝐶𝑠

X-ray diffraction experiments: Powder X-ray diffraction (XRD) data, for crystalline phase analysis, were recorded with a RINT2500 diffractometer in Laue geometry, using filtered Cu Kα radiation. The XRD patterns were recorded in the range of 2Θ = 20°- 80° by step scanning, using 2Θ increments of 0.05° and a fixed counting time of 2 s/step. A qualitative analysis of the crystalline phase content was performed using the QUALX 2.0 program. 29

Results and Discussion


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The fabrication of SLNs loaded with Cu2-xS NCs (N-SLNs) has been performed by implementing a two-step approach. First, hydrophobic Cu2-xS NCs with plasmonic characteristics in the NIR spectral region have been synthesized

26-28

(Figure 1A), and

dispersed into the complex matrix containing lipid mixtures in chloroform for their encapsulation into the SLN matrix (Figure 1B). Then, a two-phase system has been formed by adding pure water. N-SLNs dispersed into an oil-in-water emulsion have been obtained by means of hot homogenization-evaporation technique 17-18 and after complete evaporation of the organic phase, an aqueous suspension of N-SLNs has been successfully prepared. (Figure 1 B). The control on the optical properties of the Cu2-xS NCs, representing the photoactive domains in the proposed lipid based nanovectors, is fundamental for their expected PPTT applications. Indeed, the plasmonic properties of the Cu2-xS NCs, arising from the charge densities of holes associated with cation vacancies able to generate the plasmon band in the NIR, depend on the material stoichiometry, and hence material phase, size and surface chemistry. 30


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Figure 1. Pictorial scheme of the synthesis of Cu2-xS NCs (A) and for the preparation of solid lipid nanoparticles loaded with Cu2-xS NCs (B). The scheme does not show the proper proportion of the depicted structures.

Consequently, the choice of a suitable synthetic procedure for colloidal Cu2-xS NCs represents a crucial step. Here, hot injection strategy based on thermal decomposition of precursors in coordinating amphiphilic ligands solution and further injection of precursor at high temperature has been used to prepare colloidal hydrophobic Cu2-xS NCs capped with amphiphilic OLA/OA molecules. Figure 2 reports the morphological, spectroscopic and structural characterization of copper deficient Cu2-xS NCs synthesized by the hot


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injection of equimolar solution of S in OA/ODE into CuCl dissolved in OA/OLA/ODE at 180°C. Highly monodispersed spherical NCs of nearly 5 nm (σ% 11 %, Figure 2A, B), easily dispersed in organic solvent have been synthesized under this reaction condition. The UV-Vis-NIR absorption spectrum (Figure 2C) shows an intense absorption in the UV region ascribed to direct band gap of the semiconductor, and an intense LSPR band in the NIR region, between 700-2000 nm, covering the “second biological transparent window” with a resonance energy sp of 1.06 eV (1167 nm) and a full width at half maximum (FWHM ,  ) of 0.74 eV (Figure 2C). Powder X-ray diffraction (XRD) analysis (Figure 2D) shows a pattern characterized by peaks, that, although broad, as expected from nanoscopic size of the crystalline domains, could be indexed with crystallographic form of cubic digenite (Cu2-xS, Crystal System: cubic; PDF2-ICDD code 21292

31).

Additional reflexes could be detected and safely ascribed to the oxide surface layer of CuO (CuO, Crystal System: monoclinic; PDF2-ICDD code 481548

32).

Several features

in the synthetic step can be considered responsible of the formation of such digenite pdoped Cu2-xS NCs.


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Elemental sulphur, used as sulphide precursor, readily dissolves in OA at 150°C

33

where it turns into persulphides. Such a specie acts as precursor and, once injected in the copper solution, stabilizes the highly copper deficient covellite phase.34 The use of OLA/OA ligand mixture, characterized by a slight excess of OLA, results advantageous for dispersing CuCl precursor and for controlling NCs size and optical properties during the synthesis. The OLA indeed ensures (i) the copper chloride dissolution, that takes place thanks to the strong coordination ability to copper ions, (ii) the coordination of NC surface, and (iii) the occurrence of the Cu(I) oxidation to Cu(II), that contributes to enhance the self p-doping, increasing the high charge carrier density.26


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Figure 2. TEM micrograph (A), statistical size analysis (B), and UV-Vis-NIR spectroscopic (C, sketches of the hydrophobic NCs and picture of the colloidal solution, inset) and XRD characterization (D) of Cu2-xS NCs synthesized by hot injection at 180°C, reaction time 15 min (blue bar: CuO phase, PDF2-ICDD code 481548; red bar: Cu2-xS phase, PDF2ICDD code 21292)

This last OLA-induced effect is able to counterbalance the influence of OA that, also present in the reaction mixture and at the NC surface, may trap, surface holes, in form of deprotonated carboxylate, thereby effectively reducing the carrier concentration.27,

28

Finally, the used Cu:S equimolar ratio can be also considered to favour the formation of high copper deficient Cu2-xS NCs, such as digenite (Cu1.8S) up to covellite (CuS), that show the intense LSPR band. Starting from the as prepared colloidal dispersion of digenite Cu2-xS NCs in organic solvent, the encapsulation of the NCs into the SLNs resulted in a nanoformulation (NSLNs) successfully dispersible in aqueous medium, preserving the NC plasmonic


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properties. The used NCs encapsulation procedure is based on hot homogenizationevaporation of an oil-in-water emulsion. Namely, a mixture of lipids and the amphiphilic coated NCs in chloroform has been prepared and homogenized. Lipid ingredients consisting of cholesteryl oleate, triglyceride, cholesterol, DOPE, DC-chol and DSPE-PEG have been used to obtain cationic LDL-mimicking SLNs, that are convenient to delivery of hydrophobic chemotherapeutic agents embedded in their internal lipid core and/or of anionic siRNA electrostatically complexed on their surface.18,

35

The resulting LDL-like SLNs are

characterized by a core/shell structure and, namely, they are composed of a core constituted of cholesteryl oleate and triglyceride and an amphiphilic shell made of DOPE, DSPE-PEG, free cholesterol and DC-chol. Both DOPE and DC-chol induce the formation of cationic SLNs, being cationic lipids, while cholesterol provide morphological rigidity at the surface of the SLNs. Indeed, DSPE-PEG macromolecules, characterized by a hydrophobic phospholipid functionality and a hydrophilic head made of PEG, ensure an enhanced stability of cationic SLNs in physiological environments via the steric stabilization effects of PEG.36


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Figure 3. UV-Vis-NIR spectroscopic characterization of the empty SLNs (grey line), SLNs loaded with Cu2-xS NCs (wine line) in aqueous media and the as prepared Cu2-xS NCs (black dashed line). Picture of aqueous dispersion of the empty (B) and the NCs containing SLNs (C), respectively along with their schematic sketch.

The amphiphilic molecules at the Cu2-xS NCs surface represent the interface able to protect them and, at the same time, to drive the formation of a homogeneous dispersion, by means of hydrophobic interactions among ligand alkyl chain and the hydrophobic sites of the lipids in the mixture. Once entrapped in the hydrophobic core of the SLNs, the plasmonic NCs result sheltered from the external aqueous medium.


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UV-Vis-NIR spectroscopic characterization of the final aqueous suspensions is reported in Figure 3A (wine line). The suspension of SLNs loaded with Cu2-xS NCs (N-SLN, Figure 3C) shows the characteristic brown color of Cu2-xS NCs (Figure 2C, Inset), while empty SLNs (E-SLNs, Figure 3B), prepared as blank experiment, appears as a milky suspension. Although the spectrum of the E-SLNs shows a wide and broaden tail in the UV-Vis region (Figure 3A grey line), ascribed to the carbonyl and non-saturated bonds of the lipid components, and a faint absorption peak at 1400 nm due to the overtone band of -OH stretching vibration of cholesterol, it is almost transparent over the near and mid infrared region of the electromagnetic spectrum. The absorbance spectrum of the N-SLNs (Figure 3A, wine line) confirms that the spectroscopic properties of Cu2-xS NCs, and in particular its LSPR, are successfully conveyed into the hybrid nanosystem, without any appreciable change in the spectral line shape with respect to the as prepared Cu2-xS NCs (Figure 3A, dashed line) with the exception of some scattering and a slight increase in the absorbance intensity at high energy, that can be ascribed to contribution of the SLN matrix absorbance in the UV-Vis region.


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Morphological characterization of E-SLNs and N-SLNs has been performed by DLS and TEM investigation (Figure 4). Namely, DLS analysis clearly highlights a monomodal size distribution for both the E-SLNs (Figure 4A) and N-SLNs (Figure 4B), with hydrodynamic diameter values of 112 ± 3 nm (PDI 0.196 ± 0.024) and 145 ± 1 nm (PDI 0.217 ± 0.016), respectively (Figure 4). The low PDI values prove that no formation of aggregates occurs, both for the empty and the NCs loaded aqueous nanoformulations.

Figure 4. Size distribution by intensity and TEM micrographs with negative staining of aqueous suspension of SLNs (A-A2) and NCs containing SLNs (B-B2).


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The larger average hydrodynamic diameter of N-SLNs is consistent with the presence of multiple Cu2-xS NCs encapsulated within a single SLN, as demonstrated by the TEM investigation (Figure 4B1-B2). In particular, TEM analysis of N-SLNs, performed with negative staining, revealed at low contrast the occurrence of spherical nanostructures, each containing round shaped assemblies of Cu2-xS NCs, at higher contrast (Figure 4 B1B2). The micrograph at higher resolution (Figure 4B2) shows that the NCs still preserve their individuality. Depending on the number of NCs hosted in the SLN, the size varies from 30 nm, when only few (about 10) Cu2-xS NCs are embedded, to 130 nm, when a much larger amount (about 150 nm) of NCs is encapsulated into a single SLN. Conversely, the TEM micrographs of the E-SLNs show spherical empty nanostructures, with a low contrast and characterized by diameter values ranging from 25 to 100 nm (Figure 4 A1-A2). The TEM analysis confirms the findings obtained by DLS investigation passing from empty to NC loaded SLNs, taking also into account the shrinkage of the SLNs occurring during the drying process preliminary to the TEM analysis. ζ-potential measurements performed on the aqueous suspension containing lipid based nanoformulations have resulted in ζ potential values of (+62.0 ± 0.3) mV and (+61.5 ±


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0.5) mV for the empty SLN and N-SLN samples, respectively, thus highlighting their extremely high colloidal stability in aqueous medium. Moreover, the positive ζ potential values confirmed that lipids in the SLNs are organized in such a way to expose as outermost layer the cationic components, while TEM characterization shows that the hydrophobic NCs are mainly hosted in the core of the SLN nanostructures (with an average number of 57 NCs per SLN, estimated by TEM). The high positive charge recorded on the resulting plasmonic and cationic LDL mimicking-SLNs is a peculiar feature for the future preparation of the specific siRNA/SLNs complexes useful as nanovectors for gene delivery. The encapsulation efficiency of NCs (EE%) in the SLNs has been also determined, as described in the experimental section, with a value higher than 90%. The overall results demonstrated the success of the synthetic route in fabricating the plasmonic nanoformulation, with high colloidal stability in aqueous medium and average hydrodynamic diameter lower than 150 nm, highlighting the noteworthy ability of SLN of being loaded with hydrophobic NCs.


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Theoretical calculation of Cu2-xS NC and N-SLN plasmonic parameters and concentration from spectroscopic and morphologic measurements. When electromagnetic radiation is incident on a plasmonic nanoparticle, photons can be absorbed by the NCs and transformed into heat, or scattered in all directions. Both processes can be quantified by considering the absorption (σabs) and scattering (σsca) cross sections, being their sum the extinction cross section (σest, Equation 1) that characterizes the total loss that the incident radiation undergoes

 ext   abs   sca

(1)

In the case of the Cu2-xS NCs, the absorption spectrum is dominated by the LSPR.37 The quasi-static Drude model applied to Mie scattering theory, used with good results to study plasmonic noble metal NPs 38 allows to determine the free carrier density also for copper chalcogenide NPs. 26, 30, 39-43 Here, the experimental NIR absorption spectra were fitted on the basis of Maxwell’s equation solved according to Mie theory and Drude model for plasmonic NPs. This approach not only allows to determine plasmonic parameters and the free carrier density and Cu deficiency, which finally depict the stoichiometry and


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crystalline phase of the as synthesized materials, but ultimately allows to calculate, through the spectroscopic LSPR fingerprint and TEM analysis, the concentration of NC containing SLNs (N-SLN) and Cu2-xS NCs. Such parameters are crucial for setting up in

vitro and in vivo experiments and for the evaluation of the nanoformulation as potential nanoheater by light to heat transition. From the Drude model (see Supporting Information) applied to the synthesized 5 nm sized Cu2-xS NCs, dispersed in TCE (dielectric constant  m =2.28), a bulk plasma oscillation frequency  p = 3.06 eV, hole carrier density N h = 5.311021 cm-3 and an average number of free carriers per NC nh , NC = 488 have been calculated (See Supporting Information Equation S1, S2, S3, respectively). The plasmon properties allow to define the copper deficiency Cu% =12.5% (Equation S8) and to estimate a Cu1.75S stoichiometry. It comes out that, due to the characteristic phase dependence from composition (stoichiometry) for this class of materials, the LSPR band provides the optical signature of the crystallographic phase,44 corresponding, for the investigated sample, to


26

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digenite, in agreement with the XRD characterization (Figure 2D). All these results confirm the material ability to sustain plasmon band in the NIR region. Taking into account the Mie theory, the extinction cross-section in electrostatic dipole approximation for spherical NCs with a radius r   (where  is wavelength of incident radiation), Equation 2 can be defined 46:       m  8 4 6      m  ext    4 kr Im   k r     2 m      2 m  3 3

where k  nm

 c

2

(2)

is the wave vector, r is the NC average radius,    is the complex

dielectric function of the spherical NCs and nm and  m are the medium refractive index and dielectric permittivity, respectively. For smaller NCs, indeed, the electric field extends beyond the geometrical boundary of the NC, therefore the spectral position of the LSPR band depends on the  m . Substitution in Equation 2 of the complex dielectric function, ε()= ε1() + iε2(), which is function of the frequency and is composed by a real ε1() part (related to the strength of the polarization induced by the electric field) and an imaginary ε2() part (related to the losses encountered in polarizable materials), results in a theoretical extinction cross section (Equation 3)


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4 2 2 8    2 6  1   m    2 (3)     12   r     m r 2 2 c 1  2 m    22  1  2 m    22 3  c  t ext



2

32 m

3

In general, for semiconductors that behave as low loss plasmonic materials, ε1() should be negative and ε2() small. 46 According to the Drude model, ε1() and ε2() can be replaced by the following equations:

1      

 p2 2   2

and  2   

 p2  

  2   2 

with   representing the background dielectric constant, that is considered roughly independent from the wavelength in limited spectral range. The addition of the ε contribution to the Drude free carrier response takes into account for interband transition, thus providing a more generalized description of the dielectric function. Also stated as high frequency dielectric function, ε is referred to that case where the LSPR energies are sufficiently far from interband transition.44 The value of ε has been highly debated, remaining almost unknown and usually extracted by fitting the experimental data.47 Its value can induce red shift and/or damping of the plasmon band. For Cu2-xS different dielectric functions and ε values have been retrieved depending on the material charge carrier density, and thus stoichiometry, and size. Based on the above discussion, our


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purpose has been to extracted the εꝏ value from the fitting of the experimental extinction cross section in order provide a comprehensive understanding of the plasmonic properties. In particular, a controlling variable method has been here carried out by computing the theoretical extinction cross section 𝜎𝑡𝑒𝑥𝑡 () related to ε and average diameter d and comparing this curve with the experimental extinction cross section 𝜎𝑒𝑒𝑥𝑡 (). The latter (see Equation 4) has been calculated from the absorbance spectrum profile A(), known the molar concentration C (that is 0.17 μM for the NC solution used for the spectrophotometric characterization, see Supporting Information and reference 48) e  ext   

ln10  A   b  C  N A 1017

(4)

The optimized computed spectrum reported in Figure 5 illustrates the excellent agreement between the experimental (Figure 5, black line) and theoretical (Figure 5, red line) extinction cross-section for ε = 3.8 and d= 4.8 nm. The value of ε is quite close to the value reported in literature26 for Cu2-xS with similar digenite stoichiometry (Cu1.8S), and the extracted theoretical value of the NC average diameter is in good agreement with


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the NC size as measured from statistical analysis of TEM micrographs. Under the specific resonant condition, the σext (sp) =1.6 nm2 and the  ext () = 4.2.•106 M-1 cm-1,49 which is quite close to the extinction coefficient value characteristic of a direct band gap material.

Figure 5. Experimental (black line) and theoretical (dashed red line) curves of extinction cross section (A) for as prepared Cu2-xS NCs along with the corresponding plasmonic parameters extracted from the UV-Vis-NIR spectroscopic characterization and theoretical calculations (B).

A similar approach that makes use of the Drude model to fit the experimental optical data together with morphological results has been used for the determination of the


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plasmonic features for the N-SLNs and their concentration. In order to extrapolate from Equation S1, S2, S3 and S8 in the Supporting Information the plasmonic properties of the nanosystems, it is important to provide the value of dielectric constant εm for such a complex system based on plasmonic NCs within a lipid matrix and dispersed in aqueous phase. The medium dielectric constant has been assumed that one of the cholesteryl oleate50 (  m = 2.23), since lipid rather than the water dispersant solvent can be considered as the environment surrounding the plasmonic NCs. An almost unchanged plasmon band position (sp= 1.06 eV) only slightly reduced in FWHM, γ (0.71 eV) can be measured from the absorption spectrum of the N-SLN sample (Figure 3 wine line). As a consequence, an p= 2.99 eV, a charge carrier densities (Nh=5.08  1021 cm-3) slightly decreased with respect to the free Cu2-xS NCs, a copper deficiency Cu% of 12% and a Cu1.76S stoichiometry have been calculated. This slight change in the plasmonic features can be mainly ascribed to the variation of NC environment that affects the NC surface chemistry. In particular, the ester groups of the cholesteryl oleate and glyceryl trioleate, composing the N-SLN core where the NCs are trapped,41 may interact with the


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NC surface through the deprotonated carboxylic group, partially inducing a charge trapping effect that reduces the carrier density due to coordinating of carbonyl oxygen to copper (II) atoms.

Figure 6. Experimental (blue line) and theoretical (dashed red line) curves of extinction cross section (A) for NCs containing SLNs along with the corresponding plasmonic parameters extrapolated from the UV-Vis-NIR spectroscopic characterization and theoretical calculations (B).

A controlling variable method has been exploited by comparing the 𝜎𝑒𝑒𝑥𝑡 () (Equation 4) with the 𝜎𝑡𝑒𝑥𝑡 () (Equation 3). The best fitting between the two curves (Figure 6) has


32

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been achieved by computing 𝜎𝑒𝑒𝑥𝑡 () as a function of the N-SLN concentration, CN-SLN, and 𝜎𝑡𝑒𝑥𝑡 (), as a function of   . With respect to Cu2-xS NCs, whose concentration value can be readily achieved from NC weight concentration and density of the bulk material (See Supporting Information), the CN-SLN, required for the determination of the 𝜎𝑒𝑒𝑥𝑡 () , is not straightforward. For such a complex system, based on a hybrid structure with inorganic NCs encapsulated within a mixture of lipid, that constitute the SLN matrix, therefore, an iterative approach is used by calculating the 𝜎𝑒𝑒𝑥𝑡 () for different CN-SLN values. The 𝜎𝑡𝑒𝑥𝑡 () has been determined related only to the   by using the average diameter of the plasmonic core resulting from the statistical analysis of the TEM micrograph, as input data. Here, a plasmonic core diameter of 22 nm has been measured from the core volume consisting in average number of plasmonic Cu2-xS NCs of about 57 encapsulated within the SLN and trapped in close proximity with each other. It is worth to not that, in this size regime, scattering phenomena, arising from the plasmonic core, can be considered negligible, and the main contribution to the extinction cross section comes from the absorption. (See Figure S1 in Supporting Informations).


33


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The best fitting between 𝜎𝑒𝑒𝑥𝑡 () (Figure 6 blue line) and 𝜎𝑡𝑒𝑥𝑡 () (Figure 6 red line) is achieved for CN-SLN= 85 nM and ε = 3.5 (Figure 6). The ε value is similar to that of the free Cu2-xS NCs dispersed in TCE. The concentration value CN-SLN expressed in terms of N-SLN molarity (4μM in term of Cu2-xS NCs), is in a good agreement with the concentration of the nanoformulation obtained by the morphological (DLS and TEM) analysis and lyophilization (see Supplementay Material) and corresponds to 10.15 mg/mL.

Conclusion Here, colloidal copper deficient Cu2-xS NCs with plasmonic LSPR band in the second biological window, successfully synthesized by hot injection approach, have been optically and morphologically characterized in order to be conveniently exploited as efficient nanoagents for PPTT or imaging applications. The as synthesized plasmonic NCs have been incorporated in cationic LDL-like SLNs, a novel lipid based carrier, characterized by a hydrophobic core, able to encapsulate the hydrophobic NCs, and a hydrophilic shell, that improves dispersion and stability in physiological media.


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Morphological and spectroscopic analysis have confirmed the successful encapsulation of Cu2-xS NCs in the core of lipid based nanoformulation, characterized by NC EE%˃90%, and the retention of their peculiar plasmonic properties, thanks to the optical transparency of lipid matrix over the near and mid infrared region of the electromagnetic spectrum. Remarkably, the NCs turn to be the only responsible of absorption of the excitation radiation in the NIR region, as the lipid components appear transparent therein. Finally, based on the combination of experimental data obtained by optical and morphological characterization and theoretical considerations derived from Mie theory and Drude model, the specific plasmonic parameters, such as the free carrier density, Cu deficiency, crystalline phase of the as synthesized materials, but also the extinction coefficient of the nanosystems and, ultimately, the concentration of N-SLNs in an aqueous suspension have been calculated. N-SLN concentration result nearly 85 nM, with an average of 57 Cu2-xS NCs per N-SLN. Such a procedure is able to provide an accurate estimation of the NCs concentration in the SLNs, reliably answering the need of a reliable determination of concentration in the active nanosystems, fundamental for any regulatory approval and application of new nanoformulations in clinic. The prepared


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nanostructured materials have the potential to be conveniently exploited as efficient nanoagents for PPTT or imaging applications, as well as for drug and gene delivery.

ASSOCIATED CONTENT

Supporting Information. Drude model applied to Cu2-xS and calculation of the main parameters and equations and formula related to the calculation of the Cu2-xS and SLN concentrations are reported in the Supporting Infortmation. The following files are available

free

of

charge.

brief description (file type, i.e., PDF)

AUTHOR INFORMATION

Corresponding Author *[email protected]; [email protected]

Present Addresses † Present address: Università degli Studi Milano Bicocca, Dipartimento Scienze dei Materiali Piazza dell'Ateneo Nuovo, 1 - 20126, Milano


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT

The work was partially supported by the Italian National PON TARANTO Project (ARS01_00637) and FONTANAPULIA “Fotocatalizzatori Nanostrutturati e Radiazioni UV per un’Acqua più Pulita” (CodWOBV6K5-CUPB37H170049000007, 2018-2019). REFERENCES (1) Song, X.; Chen, Q. ; Liu, Z. Recent Advances in the Development of Organic Photothermal Nano-agents. Nano Res. 2015, 8, 340-354

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