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1.
J Chem Phys ; 160(12)2024 Mar 28.
Artículo en Inglés | MEDLINE | ID: mdl-38516973

RESUMEN

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this article, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ∼6 ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50 ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.

2.
Nano Lett ; 22(13): 5230-5235, 2022 Jul 13.
Artículo en Inglés | MEDLINE | ID: mdl-35763556

RESUMEN

Black phosphorus (bP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations occur concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and bP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early time out-of-plane dynamics reported here have important implications for single- and few-layer bP and other vdW materials with strong electronic-lattice correlations.

3.
Nano Lett ; 20(3): 2026-2033, 2020 Mar 11.
Artículo en Inglés | MEDLINE | ID: mdl-32031381

RESUMEN

In this Letter, the ultrafast structural, interfacial, and carrier dynamics of monolayer MoS2 supported on sapphire are cross-examined by the combination of ultrafast electron diffraction (UED) and transient reflectivity techniques. The out-of-plane motions directly probed by reflection UED suggest a limited anisotropy in the atomic motions of monolayer MoS2, which is distinct from that of related materials such as graphene and WSe2. Besides thermal diffusion, the MoS2-sapphire interface exhibits structural dynamics trailing those of the overlaying MoS2 and are in stark contrast with the sapphire bulk, which is consistent with the limited thermal boundary conductance. These structural dynamics provide justification for the determination of carriers being trapped by defects in ∼600 fs and releasing energy within a few picoseconds. The rich findings attest to the strength of combining techniques with real-time optical and direct structure probes for a detailed understanding of dynamical processes in functional materials.

4.
J Photochem Photobiol B ; 173: 307-317, 2017 Aug.
Artículo en Inglés | MEDLINE | ID: mdl-28624736

RESUMEN

Medicinal properties of curcumin are widely getting realized. For its applicability as a hydrophobic drug molecule and food spice interaction of curcumin with rhamnolipids, a biosurfactant, bears importance. Here we have explored interaction of curcumin with rhamnolipids biosurfactant and its aggregation behavior. The impact of pH on critical micelle concentration (cmc) of rhamnolipids has been studied using fluorescence of curcumin and found that cmc of rhamnolipids increases with increase in pH of the medium. In acidic, neutral and slightly alkaline medium (pH8), at λex=355nm (for ß-diketone form) curcumin undergoes excited state hydrogen transfer (ESHT) and emits solely from enol form both in the presence and absence of rhamnolipids, but first time we report that in extreme alkaline condition, at pH13, at λex=355nm curcumin emits from both ß-diketone as well as enolic ESHT forms in absence of rhamnolipids but in the presence of rhamnolipids ß-diketone is stabilized and the emission solely comes from ß-diketone by completely revoking ESHT process. Fluorescence quenching by hydrophobic cetylpyridinium bromide confirms curcumin penetrates deep inside the hydrophobic pocket of rhamnolipid aggregates/micelle by reducing the distance between N+-atom of pyridinium ion and curcumin. On the other hand hydrophobic molecule like pyrene stays near to the Stern layer of rhamnolipids facilitating electron transfer from pyrene to N+-atom of pyridinium ion. Even in neutral condition, in the presence of rhamnolipids the ß-diketone form, though in small proportions, can be stabilized in higher temperature in expense of enolic ESHT form, thus, offering an on off ratiometric fluorescence temperature sensing in solution, which bears significance as ratiometric probe molecules. Interaction of curcumin with rhamnolipids stabilizes curcumin in acidic, neutral and moderate alkaline condition but fails at extreme pH13.


Asunto(s)
Curcumina/química , Glucolípidos/química , Tensoactivos/química , Temperatura , Concentración de Iones de Hidrógeno , Isomerismo , Micelas , Espectrometría de Fluorescencia
5.
Colloids Surf B Biointerfaces ; 149: 30-37, 2017 Jan 01.
Artículo en Inglés | MEDLINE | ID: mdl-27716529

RESUMEN

Stability of curcumin in neutral and alkaline buffer conditions has been a serious concern for its medicinal applications. We demonstrate that the stability of curucmin can be improved in 1,2-Dioctadecanoyl-sn-glycero-3-phosphocholine (DSPC) liposomes. Curcumin strongly partition into liquid crystalline phase compared to solid gel phase of DSPC liposomes. Variation of fluorescence intensity of curcumin associated with liposomes with temperature successfully determines phase transition temperature of DSPC liposomes. However, at higher molar ratio curcumin can influence phase transition temperature by intercalating into deep hydrophobic layer of liposomes and facilitating fusion of two membrane phases. Rhamnolipids (RLs) are recently being applied for various biomedical applications. Here, we have explored new insight on intercalation of rhamnolipids with DSPC liposomes. Intercalation of rhamnolipids exceptionally increases partition of curcumin into solid gel phase of DSPC liposomes, whereas this increase is moderate in liquid crystalline phase. Fluorescence quenching study establishes that permeability and fluidity of the DSPC liposomes are enhanced in the presence of RLs. Membrane permeability and fluidity can be improved further by increasing the percentage of RLs in DSPC liposomes. The phase transition temperature of DSPC liposomes decreases with increase in percentage of RLs in DSPC liposomes by encouraging fusion between solid gel and liquid crystalline phases. Intercalation of RLs is found to further boost stability of drug, curcumin, in DSPC liposomes. Thus, mixing RLs with DSPC liposomes could potentially serve as a good candidate for drug delivery application.


Asunto(s)
Curcumina/química , Decanoatos/química , Liposomas/química , Lisofosfatidilcolinas/química , Ramnosa/análogos & derivados , Sistemas de Liberación de Medicamentos , Estabilidad de Medicamentos , Interacciones Hidrofóbicas e Hidrofílicas , Cinética , Fluidez de la Membrana , Permeabilidad , Transición de Fase
6.
Talanta ; 169: 104-114, 2017 Jul 01.
Artículo en Inglés | MEDLINE | ID: mdl-28411798

RESUMEN

As a proof of concept, here it is established that curcumin integrated chitosan oligosaccharide lactate (COL) self-assembles on silica nanoparticle surface to form nano hybrid particles (NHPs). These NHPs have size in the ranges of 25-35nm with silica nanoparticle as its core and curcumin-COL as outer layer having thickness of 4-8nm. The fluorescence intensity of these NHPs are found to be quenched and emission maximum is ~50nm red shifted compared to free curcumin implying inner filter effect and/or homo-FRET between curcumin molecules present on the surface of individual nano hybrid particle. Although fluorescence of free curcumin is remarkably quenched by Hg2+/Cu2+ ions due to chelation through keto-enol form, the fluorescence of NHPs is unaffected by Hg2+/Cu2+ ion that boosts analytical selectivity. The fluorescence intensity is outstandingly enhanced in the presence of cholesterol but is not influenced by ascorbic acid, uric acid, glucose, albumin, lipid and other potential interfering substances that either obstruct during enzymatic reaction or affect fluorescence of free curcumin. Thus, NHPs outstandingly improve analytical specificity, selectivity and sensitivity during cholesterol estimation compared to free curcumin. The interaction between cholesterol and NHPs is found to be a combination of ground state electrostatic interaction through the free hydroxyl group of cholesterol along with hydrophobic interaction between NHPs and cholesterol and excited state interaction. The proposed cholesterol biosensor illustrates a wider linear dynamic range, 0.002-10mmolL-1, (upper limit is due to lack of solubility of cholesterol) needed for biomedical application and better than reported values during enzymatic reaction. In addition, the NHPs are found to be photo-stable potentially making it suitable for simple, quick and cost-effective cholesterol estimation and opening an alternative approach other than enzymatic reaction using nano hybrid structure to tune analytical specificity, selectivity and sensitivity of probe molecule.


Asunto(s)
Técnicas Biosensibles/métodos , Colesterol/análisis , Curcumina/química , Nanopartículas/química , Polielectrolitos/química , Dióxido de Silicio/química , Antineoplásicos/química , Humanos , Interacciones Hidrofóbicas e Hidrofílicas
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