Hybrid nanoparticles as photoinitiators

Abstract

Provided is a novel photoinitiator in the form of a hybrid nanoparticle constructed of a semiconductor and metallic regions, and uses thereof.

Claims

1. A method for generating a reactive species in the form of a radical or a peroxide in a medium, the process comprising irradiating a medium comprising hybrid nanoparticles (HNPs) and at least one polymerizable material susceptible of polymerization, wherein each of said HNP comprises at least one metal/metal alloy region and at least one semiconductor region.

2. The method according to claim 1, wherein the medium is a printed pattern.

3. The method according to claim 1, wherein said pattern is achieved by inkjet printing.

4. The method according to claim 3, wherein irradiation is carried out after, during, or concomitant with the patterning steps or after several layers have been deposited or after the full object has been formed.

5. The method according to claim 1, wherein the HNPs comprise at least two populations of nanoparticles, each population being reactive under light of a different wavelength.

6. The method according to claim 1, wherein the HNPs have at least one elongated structure element comprising a semiconductor material, bearing on at least one end portion thereof a material selected from metal and metal alloy.

7. The method according to claim 6, wherein semiconductor material is selected from Group II-VI semiconductors, Group III-V semiconductors, Group IV-VI semiconductors, Group IV semiconductors, Group III-VI semiconductors, Group I-VI semiconductors, ternary semiconductors, and alloys of any of the above semiconductors; or as combinations of the semiconductors in composite structures and core/shell structures.

8. The method according to claim 1, wherein the HNPs are in a form selected from dots, rods, platelets, tetrapods, frames, and nanodumbells, each form comprising at least one metal/metal alloy region and at least one semiconductor region.

9. The method according to claim 8, wherein the HNPs are nanodumbells.

10. The method according to claim 1, wherein the medium is a liquid or solid medium.

11. The method according to claim 10, wherein the medium is water or an aqueous medium.

12. The method according to claim 1, wherein the medium is a biological medium.

13. The method according to claim 10, wherein the solid medium is a solid object.

14. The method according to claim 1, for polymerizing at least one polymerizable material, the method comprising irradiating the medium comprising the HNPs and the at least one material susceptible of polymerization.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1A-B show optical and structural characterization of HNPs along with their HRP photocatalytic activation. FIG. 1A—Absorption spectra of CdSe/CdS NRs and CdSe/CdS—Au HNPs, showing similar features. The contribution of the small Au tips is mainly manifested in the small increase of the absorption tail toward the red. Inset-TEM image presenting matchstick-like structure of 52±4 nm×4.4±0.4 nm CdSe/CdS nanorods with small gold tips on their apex (stronger contrast, 1.6 nm in diameter). FIG. 1B—Staircase behavior of HRP stimulation while turning off and on the excitation source, indicated by black and grey arrows, respectively. (Inset) Detection of HRP product by absorbance measurements with its characteristic peak around 495 nm, growing with increased irradiation times of the HNP and HRP solution.

(3) FIGS. 2A-H show TEM images of hybrid nanorods CdSe/CdS—Au (52 nm×4.4 nm) (FIG. 2A) and CdS—Au (49 nm×4.2 nm) (FIG. 2E). Sizing histograms for rods diameter (FIGS. 2B,F), length (FIGS. 2C,G) and Au metal domain diameter (FIGS. 2D,H).

(4) FIG. 3 provides absorption spectra of CdSe QDs and CdSe/CdS NRs. Inset showing the CdSe seed feature within the seeded rod structure.

(5) FIGS. 4A-B provide: FIG. 4A—Reaction scheme for production of quinoneimine catalyzed by HRP in the presence of 4-AAP and phenol upon light stimulation of HNPs. FIG. 4B—Control measurements of photo-induced modulation of HRP activity by HNPs were done in various conditions, absence of enzyme or oxygen, without irradiation and absence of HNPs. The result of the full assay which includes all these parameters is shown in the black curve.

(6) FIGS. 5A-C provide comparative study of product formation by HRP activation using HNPs, NRs, and QD seeds in different conditions. FIG. 5A—Light stimulation of hybrids in the presence of SOD and ethanol results in 3- and 15-fold higher product formation after 500 s, than when stimulating bare nanorods or CdSe seeds, respectively. FIGS. 5B-C—Comparative bar charts for the HRP activity in different conditions, plotting product formation after 500 s (note the different scale in FIGS. 5B-C). Addition of SOD to the solution increases the efficiency of all of the tested nanoparticle systems and the efficiency is further enhanced upon adding the hole acceptor (HS), which is ethanol in this case. Notably, the hole acceptor and SOD effects on the efficiency of the HNPs system was much more pronounced compared to the NRs and QDs.

(7) FIG. 6 shows comparison of HRP activity upon light irradiation between CdS—Au HNPs and CdS NRs in the presence of SOD and hole scavenger (e.g., EtOH) and in the absence of these additives.

(8) FIGS. 7A-D show HNPs photocatalytic ROS formation mechanism. FIG. 7A-Kinetic measurements of molecular oxygen consumption by HNPs and NRs coated with PEI upon light illumination using polarography. FIG. 7B—Kinetic measurements of hydroxyl radical formation using the fluorescence TPA assay (inset). Note the significantly faster and more efficient hydroxyl radicals production upon stimulation of HNPs in comparison to NRs. Addition of ethanol as a hole scavenger to the hybrids solution prevented this reaction, yielding similar signals to those of control TPA alone. FIG. 7C—EPR measurements following excitation of HNPs (upper panel) and NRs (lower panel), and corresponding fits. The results show near-solely (90%) superoxide production for the bare NRs, whereas the HNPs signal reveals additional peaks which are attributed to significant hydroxyl radicals production. FIG. 7D—Time-resolved measurements for the superoxide radical signal at static magnetic field of 3448G (star mark in panel c) show higher superoxide signal buildup for NRs over HNPs, followed by a plateau when light is on (grey arrows), resulting from balanced formation and destruction of the radicals. Fast decay of the radical signal is seen upon turning off the excitation source (black arrows).

(9) FIG. 8 shows EPR measurements during illumination of HNPs or NRs (upper and lower panels, respectively) in the presence of ethanol resulted in decreased signal of DMPO-OH and in increase in signal with hyperfine coupling of aN=15.8G and aH=22.8G which indicates the presence of CH3C*HOH, an ethanol-derived radical adduct. This confirms the DMPO-OH results from the presence of hydroxyl radicals in the solution.

(10) FIG. 9 provides a summary scheme showing the different pathways for ROS formation after HNP light activation, and their use for HRP modulation. Excitation of the semiconductor rods results in charge separation followed by reduction of molecular oxygen by the excited electrons. This results in direct formation of H.sub.2O.sub.2 that can be used as a substrate for HRP or in formation of superoxide that can be converted to H.sub.2O.sub.2 with the aid of SOD. In parallel, the holes can be used to produce hydroxyl radicals or could be scavenged by hole acceptors.

(11) FIGS. 10A-C depict HNPs surface effects and biocompatibility properties. FIG. 10A-HRP activity upon light stimulation of HNPs with different surface coatings in the presence of SOD and ethanol. HNPs coated with PEI show 5-fold higher efficiency in comparison to thiolate ligands, such as GSH and MPA. PSMA showed the lowest efficiency. FIG. 10B—MTT viability assay with cultured K-562 cells shows that their incubation for 24 h with different concentrations of NRs and HNPs did not significantly affect their viability. FIG. 10C—Live and dead assay 24 h after illumination on cells incubated with 2.5 and 0.5 nM of HNPs (left and right, respectively). Higher HNPs concentration shows significant cells' death (non-bright cells in the figure), an outcome that indicates a potential use for photodynamic therapy. Lower HNPs concentration showed mostly live cells (bright cells), suggesting cells' viability under light-controlled ROS production, scale bar is 100 μm.

(12) FIGS. 11A-C provide: FIG. 11A—Kinetic measurements of molecular oxygen consumption by HNPs and NRs coated with GSH upon light illumination, measured by polarography. FIG. 11B—Comparison of HRP activity upon light irradiation using the HRP activity assay between CdSe/CdS—Au HNPs and CdSe/CdS NRs coated by GSH in the presence of SOD and hole scavenger (e.g. EtOH) and in the absence of these additives. FIG. 11C—Kinetic measurements of hydroxyl radical formation using the TPA assay showing faster hydroxyl radical production upon stimulation of hybrids in comparison to bare nanorods. Addition of ethanol as a hole scavenger to the hybrids solution prevents this reaction, yielding similar signals to those of control TPA alone.

(13) FIGS. 12A-C provide: FIG. 12A—The production of HRP's product after stimulating HNPs in serum or in buffer with or without addition of hole acceptor to the solution. The results demonstrate the ability to use HNPs stimulation with and without hole acceptor for efficient and controlled production of hydrogen peroxide also in biological systems. FIG. 12B—Live/Dead assay confirms incubation of K-562 cells with HNPs for 24 hr under dark conditions didn't affect their viability (most cells are stained in green, bright in the figure), Scale bar is 100 μm. FIG. 12C—The activity of the cholinesterase enzymes is inhibited following light-induced production of ROS by HNPs but not by the irradiation itself.

(14) FIG. 13 shows kinetic measurements of hydrogen production with different biological molecules as hole acceptor agents in presence of CdS—Au HNPs (PEI coated).

(15) FIGS. 14A-C show polymerization kinetics: percent conversion of vinyl bonds calculated using aqueous acrylamide solutions with nano-photoinitiator catalysts (hybrid CdS—Au Nanorods stabilized with PEI, 6E-7 M) at 988 cm.sup.−1 (assigned to out-of-plane bending mode of the ═C—H unit) normalized to the C═O stretching peak at 1654 cm.sup.−1 as an internal standard, at varying UV (385 nm) exposure duration. FIG. 14A—CdS—Au show higher polymerization efficiency than bare CdS. FIG. 14B and FIG. 14C present light-induced polymerization using diverse CdS—Au HNPs concentration and excitation wavelengths.

(16) FIGS. 15A-B show a 3D printed hydrogel with an buckyball architecture, prepared using nano-photoinitiator catalysts (Hybrid CdS—Au Nanorods stabilized with PEI) according to the invention. The 3D printed hydrogel was prepared with CdSe/CdS NRs in the formula resulting in fluorescence architecture, providing an example of a 3D printed material that becomes functional by a second excitation wavelength.

(17) FIG. 16 demonstrates photocatalytic reduction of methylene blue (MB) by HNPs-hydrogel system. MB absorption before irradiation, reductant MB after 10 min of 405 nm illumination under aerobic and inert conditions. This provides an example for the use of HNPs as multi-functional material used both as photoinitiator and photocatalysts.

(18) FIGS. 17A-C presents TEM images. FIG. 17A presents an image of ZnSe—Au HNPs as an example for Cd free system that can be used for applications requiring the light-induce ROS formation. FIGS. 17B-C present images of a mixture of acrylamide monomers with ZnSe—Au NPs, before and after UV exposure. Upon the radiation, the liquid drop polymerized forming gel structure.

DETAILED DESCRIPTION OF EMBODIMENTS

(19) The invention discloses high-performance photo-initiators in the form of hybrid nanoparticles (HNPs) which may be used in a variety of applications, ranging from biological and chemical applications to industrial applications.

(20) The HNPs are light-activated hybrid nanoparticles comprising each at least one metal/metal alloy region and at least one semiconductor region having an absorption onset in the UV (200-400 nm), or the visible (400-700 nm) or the near infrared (NIR) range (0.7-3 μm). In some embodiments, the at least one semiconductor region has an absorption onset in the range of 350 nm to 3 μm. In some other embodiments, the at least one semiconductor region has an absorption onset in the range of 450 nm to 3 μm. In further embodiments, the at least one semiconductor region has an absorption onset in the range of 470 nm to 3 μm. In still other embodiments, the at least one semiconductor region has an absorption onset in the range of 500 nm to 3 μm.

(21) In some embodiments, the HNPs comprise at least one metal/metal alloy region and at least one semiconductor region having an absorption onset in the UV (200-400 nm, in some embodiments above 350 nm, in some embodiments above 380 nm), or the visible (400-700 nm, in some embodiments above 420, or above 450 or above 500 nm) to near infrared (NIR) range (0.7-3 μm), said nanoparticle being capable of forming, upon irradiation (illumination) with a radiation in the visible and/or NIR range, an electron-hole pair at the metal/semiconductor interface and subsequently undergo charge separation. In certain embodiments, where the nanoparticles have elongated shape, they may be prepared as disclosed in WO 05/075339, or a US application derived therefrom, herein incorporated by reference. However, the shape and size of the nanoparticle so defined may vary and is not restricted to the elongated structure.

(22) In some embodiments, the HNPs comprise at least two metal/metal alloy regions, separated by at least one semiconductor region, wherein each of said at least two metal/metal alloy regions is of a different or same metal/metal alloy material. In some embodiments, each of said at least two metal/metal alloy regions is of a different metal/metal alloy material (having different Fermi potentials). In some embodiments, the two metal/metal alloys are of the same metal/metal alloy material.

(23) In other embodiments, the HNPs comprise at least two metal/metal alloy regions, separated by at least two semiconductor regions, wherein each of said at least two metal/metal alloy regions is of a different or same metal/metal alloy material, and each of said at least two semiconductor regions having a different energy gap and/or different energy band positions.

(24) In some embodiments, the at least two semiconductor regions are separated by at least one metal/metal alloy region. In other embodiments, said at least two semiconductor regions are not separated by one or more metal/metal alloy region and are therefore referred to herein as “sub-regions”. The two or more semiconductor sub-regions are each of a different semiconductor material.

(25) Within the context of the present invention, the term “material” refers to a solid substance of which the nanoparticles or any one region thereof is made. The material may be composed of a single substance, e.g., elements, alloys, oxidized forms, etc, or a mixture of such substances, at any ratio.

(26) The HNPs employed by the methods of the invention, are discrete entities wherein at least one of its dimensions (e.g., diameter, length, etc) is between 1-20 nm. They may have rod-like structures having lengths of below 400 nm, preferably below 200 nm.

(27) Notwithstanding the above, the HNPs can have any shape and symmetry, and may display branched and net structures. Without being limited thereto, the HNPs may be symmetrical or unsymmetrical, may be elongated having rod-like shape, round (spherical), elliptical, pyramidal, disk-like, frame structure, branch, network or have any irregular shape.

(28) In some embodiments, the HNPs are nanorods having elongated rod-like shape. In some other embodiments, the nanorods are constructed of a semiconducting material having at one or both ends a metal or metal alloy region.

(29) The plurality of HNPs refers to a population of nanoparticles optionally having a narrow size distribution, shape distribution and/or a spatial arrangement, namely the arrangement of the metal/metal alloy region in relation to the semiconductor region and/or the spatial distribution of the metal/metal alloy regions on the surface of the semiconductor material. In some embodiments, the population of HNPs is not homogenous but rather tailored to sequential processes, e.g., a step-by-step process wherein each step utilizes a different population of HNPs.

(30) The HNPs comprise a semiconducting material selected from elements of Group II-VI, such as CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, HgS, HgSe, HgTe and alloys thereof such as CdZnSe; Group III-V, such as InAs, InP, GaAs, GaP, InN, GaN, InSb, GaSb, AlP, AlAs, AlSb and alloys such as InAsP, CdSeTe, ZnCdSe, InGaAs; Group IV-VI, such as PbSe, PbTe and PbS and alloys thereof; Group such as InSe, InTe, InS, GaSe and alloys such as InGaSe, InSeS; Copper chalcogenides such as CuS, Cu.sub.2S and other stoichiometry as in Cu.sub.2-xS with x ranging from 0 to 1; semiconductors such as CuInS.sub.2 and CuInxGa.sub.1-xSe.sub.2; Oxides such as ZnO, TiO.sub.2, In.sub.2O.sub.3, CuO, Cu.sub.2O, and others; Group IV semiconductors, such as Si and Ge alloys thereof, and combinations thereof in composite structures and core/shell structures. In some embodiments, the HNPs comprise semiconducting materials selected from Group II-VI semiconductors, alloys thereof and core/shell structures made therefrom. In further embodiments, the Group II-VI semiconductors are CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, alloys thereof, combinations thereof and core/shell, core multi-shell layered-structures thereof.

(31) The metal/metal alloy materials are typically transition metals. Non-limiting examples of such are Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe, Ti, Zn, Ir, W, Mo, and alloys thereof.

(32) In some embodiments, the metal is Au, Pd, and Pt and alloys thereof.

(33) In further embodiments, the metal is Au, Pd, and Pt and alloys thereof and said at least one semiconductor material is CdS, CdSe or CdTe.

(34) As stated above, the HNPs population may have a relatively narrow size distribution, namely they are manufactured or collected in a relatively narrow range of sizes. In fact, the standard deviation (sigma) of the particles' size in a single population may be less than 25%. In some embodiments, the deviation in the particles size is less than 15%. Where the nanoparticles are elongated (nanorods) the sigma of the length of a single population may be less than 35% and the sigma of the width is less than 15%. In some embodiments, the population of nanoparticles is homogenous in that said population comprises nanoparticles of relatively the same size and/or shape.

(35) For certain applications it may be desirable to vary not only the size and shape of the HNPs, making up the population, but also the chemical composition of the nanoparticles and/or the arrangement of the semiconductor and metal/metal alloy regions along the nanoparticles. Thus, in some embodiments, the population of HNPs is a blend of one or more of the following types/groups of nanoparticles: HNPs of a certain pre-determined size distribution; HNPs of a certain pre-determined shape; HNPs having one metal/metal alloy region and one semiconductor region (optionally having one or more sub-region of different semiconducting materials); HNPs having at least two metal/metal alloy regions and a single semiconductor region (optionally having one or more sub-region of different semiconducting materials); HNPs having one metal/metal alloy region and at least two semiconductor region (optionally having each one or more sub-region of different semiconducting materials); HNPs having at least two metal/metal alloy regions and at least two semiconductor regions (optionally having one or more sub-region of different semiconducting materials), HNPs having at least two metal/metal alloy regions and at least two semiconductor regions (optionally having one or more sub-region of different semiconducting materials), wherein the arrangement (sequence) of regions or sub-regions along the nanostructure differs from one population to another; HNPs which may be photoactivated at only a particular wavelength or at a only predetermined wavelength or range of wavelengths; HNPs which do not undergo photoactivation as described herein.

(36) The population of HNPs may be attained by mixing together one or more of the above types of nanoparticles. Alternatively, heterogeneous populations may be prepared by employing, e.g., non-stoichiometric amounts of starting materials. Each group of HNPs may be manufactured separately and stored for future use. As a person skilled in the art would realize, each of the above groups of HNPs may be prepared in a substantially uniform or homogenous fashion. However, due to random defects having to do with e.g., the manufacture process, purity of starting materials and other factors, a certain degree of HNPs having defects in size, shape, chemical composition, and other parameters, may be found in each of these types of HNPs. It should be noted that the presence of such defects does not necessarily reflect on any one of the herein disclosed characteristics.

(37) A population of HNPs may comprise a blend of HNPs of one or more of the above types, in a known pre-determined ratio of nanoparticles or comprise a random mixture of such HNPs. In a certain non-limiting example, a population of HNPs comprises HNPs having a large variety of sizes and shapes, constructed of a single metal/metal alloy region and two semiconductor regions (optionally having one or more sub-region of different semiconducting materials). In another example, a population of HNPs may comprise HNPs of different shapes and different chemical compositions. In yet another example, the population comprises a blend of nanorods having at least one metal/metal alloy region at one or both ends of the elongated structure and/or at least one metal/metal alloy region in a central, non-terminal part of the elongated HNPs.

(38) In addition, HNPs populations comprising any one nanoparticle according to the invention or employed in any one method of the invention, and at least one type of particle outside of the scope of the present application are also provided herein. Such mixed populations of HNPs herein described and HNPs known in the art may have advantageous effects suitable for any one application disclosed herein.

(39) As will be discussed further below, by having the ability to provide blends of different HNPs populations it is possible to tune the optical properties of the material, thus utilizing the whole range of wavelengths efficiently. Alternations in the metal composition and size allow the fine-tuning of the Fermi level energy and the redox potential of the HNPs. The different shapes enable better control and the design of a great variety of devices.

EXPERIMENTAL DESCRIPTION

(40) Chemicals: Trioctylphosphine (TOP, Sigma Aldrich, 90%) was vacuum distilled before use and stored under inert atmosphere. All other chemicals were used as purchased: cadmium oxide (>99.99%), trioctylphosphine oxide (TOPO, 99%), 1-octadecene (ODE, technical grade, 90%), oleic acid (95%), octadecylamine (ODA, ≥99%), didodecyldimethylammonium bromide (DDAB, 98%) and Gold(III)chloride (99%), Gold nanoparticles (5 nm diameter stabilized suspension in citrate buffer), L-glutathione reduced (GSH, ≥98.0%), Poly(styrene-co-maleic anhydride), cumene terminated (PSMA), sodium sulfide nonahydrate (≥98.0%), sodium sulfite (>98%), Polyethylenimine (PEI, branched average Mw˜25,000), 4-aminoantipyrine (4-AAP, ≥98.0%), Superoxide dismutase from bovine erythrocytes (BioUltra, lyophilized powder, ≥4,500 units/mg protein, ≥97%), phenol (≥99.5%), terephthalic acid (TPA, 98%), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were purchased from Sigma Aldrich. Oleylamine (technical grade, 90%) was purchased from Across. Octadecylphosphonic acid (ODPA) and hexylphosphonic acid (HPA) were purchased from PCI Synthesis. Sulfur (>99.0%) was purchased from Merck.

(41) Synthesis of CdSe Seeds:

(42) CdO (0.12 g), trioctylphosphine oxide (TOPO; 6.0 g) and octadecylphosphonic acid (ODPA; 0.56 g) were mixed in a 100 mL three-neck flask. The mixture was heated to 100° C. and placed under vacuum for 1 hour followed by three times of argon purging. Under argon atmosphere, the solution was heated to 300° C. to dissolve the CdO, forming a clear colorless solution. At this temperature, 1.8 mL of TOP was injected into the hot solution. Next, the solution was further heated to 350° C., at which 0.87 mL of precursor solution of selenium (0.21 g) in TOP (1.7 mL), was rapidly injected into the hot solution. The reaction time was, typically, 25 sec for CdSe seeds with diameter of ˜2.3 nm. The reaction was quenched by removing the heating mantle and cooling with fan. The crude reaction mixture was diluted with toluene. Methanol was added in order to precipitate the nanocrystals and remove excess surfactants.

(43) Synthesis of Seeded CdSe/CdS Nanorods:

(44) CdO (0.065 g), trioctylphosphine oxide (TOPO; 3.0 g), octadecylphosphonic acid (ODPA; 0.29 g) and hexylphosphonic acid (HPA; 0.065 g) were mixed in a 100 mL three-neck flask. The mixture was heated to 100° C. and placed under vacuum for 1 hour followed by three times of argon purging. Under argon atmosphere, the solution was heated to 300° C. and at this temperature 1.8 mL TOP was injected into the hot solution. Next, the solution was further heated to 360° C., at which a precursor solution of CdSe seeds (typically, 4×10.sup.−8 mol) with sulfur in TOP solution (1.6 mL; 0.075 g/mL) was rapidly injected. The temperature decreased and then recovered within 1-2 min. The reaction time was, typically, 12 min. The reaction was quenched by removing the heating mantle and cooling with fan. The crude reaction mixture was diluted with toluene. Methanol was added in order to precipitate the nanocrystals and remove excess surfactants. Absorption spectra of both CdSe seeds and CdSe/CdS nanorods (NRs) are presented in Supplementary FIG. 3 including the CdSe seeds feature within the seeded rod structure (inset).

(45) Synthesis of CdS Seeds:

(46) CdS nanocrystal seeds were synthesized by a modification of a previously reported procedure. Cadmium oxide (CdO; 0.106 g), oleic acid (OA; 2.26 g) and 1-octadecene (ODE; 20 g) were mixed in a 100 mL three-neck flask. The mixture was heated to 100° C. and placed under vacuum for 1 hour followed by three times of argon purging. Under argon atmosphere, the solution was heated to 260° C. to dissolve the CdO, forming a clear colorless solution. A precursor solution consisting of sulfur (0.013 g) and ODE (7 mL) was rapidly injected into the hot solution. The reaction time was typically 90 sec for CdS seeds with diameter of ˜3.1 nm. The reaction was quenched by removing the heating mantle and cooling with fan. The crude reaction mixture was precipitated with acetone followed by centrifugation. For further purification, the particles were dissolved in toluene and the precipitation procedure was repeated several times.

(47) Synthesis of CdS Nanorods:

(48) CdS nanorods were synthesized by a modification of a previously reported procedure employing seeded growth. CdO (0.12 g), trioctylphosphine oxide (TOPO; 6.0 g), octadecylphosphonic acid (ODPA; 0.68 g) and hexylphosphonic acid (HPA; 0.04 g) were mixed in a 100 mL three-neck flask. The mixture was heated to 100° C. and placed under vacuum for 1 hour followed by three times of argon purging. Under argon atmosphere, the solution was heated to 350° C. and at this temperature trioctylphosphine (TOP; 1.8 mL) was injected into the hot solution. Next, the solution was further heated to 365° C., at which a precursor solution of CdS seeds (typically, 3×10.sup.−8 mol) and sulfur in TOP solution (1.6 mL; 0.075 g/mL) was rapidly injected into the hot solution. The temperature decreased and then recovered within 1-2 min. The reaction time was 9 min for 49 nm×4.2 nm CdS rods. The reaction was quenched by removing the heating mantle and cooling with fan. The crude reaction mixture was diluted with toluene. Methanol was added in order to precipitate the nanocrystals and remove excess surfactants.

(49) Synthesis of CdS—Au and CdSe/CdS—Au Hybrid Nanorods:

(50) A precursor stock solution consisting of octadecylamine (ODA; 0.055 g), didodecylammonium bromide (DDAB; 0.021 g) and AuCl.sub.3 (0.010 g) in toluene (10 mL) was sonicated for 15 min to dissolve the AuCl.sub.3, and the solution changes color from dark brown to yellow. In order to achieve selective growth of 1.5-1.8 nm gold tips on one apex of the NRs, a molar ratio of 700-900 Au ions per NR was used depending on the specific properties of the rods. Diluted Au growth stock solution was added to NRs (typically ˜2×10.sup.−9 mol) in toluene (20 mL) in 100 mL flask under flowing argon. The solutions are mixed for 1 hour at room temperature and under dark conditions. The product hybrid nanoparticles (HNPs) are then washed and precipitated with acetone followed by separation via centrifugation. The hybrid samples had narrow size distribution according to their absorption spectra and TEM characterization measurements and statistics (FIG. 2).

(51) Synthesis of ZnSe—Au:

(52) ZnSe frames were synthesized by previously described synthesis [Jia et al., Nature Materials, 13, 301-307, (2014)]. Following purification by precipitation with methanol, For Au growth, ZnSe frames were dissolved in toluene and furthered mixed for ten minutes at room temperature with a degassed solution containing AuCl.sub.3 and oleylamine.

(53) Nanoparticle Characterization:

(54) TEM characterization was performed using a Tecnai T12 G2 Spirit and Tecnai F20 G2. All size statistics are done with “Scion image” program on 200 particles. Absorption was measured with a JASCO V-570 UV-vis-near IR spectrophotometer. Extinction coefficient values of the NRs were calculated using a previously reported method.

(55) Phase Transfer:

(56) NPs were transferred to water by ligand exchange and polymer coating methods. For exchanging the native organic-soluble ligands with the thiolate alkyl ligands, the ligand exchange strategy was used. Stock solution of L-glutathione (GSH) was prepared by dissolving GSH (140 mg) and KOH (100 mg) in methanol (1 mL). Next, 200 μL of stock solution is added to NPs in chloroform (1 mL) with an optical density of 1.5 at the CdS first state transition and mixed for 1-2 min. Based TDW (pH 11-12) is added to the flocculated solution and after mixing phase separation appears and the NPs are extracted from the upper water phase after mild centrifugation.

(57) Polymer coating was done with different polymers. Poly(styrene-co-maleic anhydride) (PSMA) coating is achieved by mixing 2 mL of nanoparticles solution with PSMA (20 mg) in chloroform (1 mL) for 5 hours. Then ethanolamine (20 μL) is added to the solution and mixed for 1-2 min. Next, recurring additions of TDW (1 mL) is done to transfer the particles to the above water phase followed by mild centrifugation before extraction. Polymer coating with polyethylenimine (PEI) was done by mixing NPs solution (1 mL) with PEI (0.15 g; MW 25,000) in chloroform (1 mL) for 1 hour. Then the particles are precipitated and washed with cyclohexane (1:1 chloroform/cyclohexane), followed by centrifugation. TDW is added to the precipitate and residues of PEI are removed by centrifugation.

(58) Before use, all nanoparticle solutions were washed through 100 KDa cellulose membrane, to remove excess of polymer and ligands.

(59) HRP Activity Assay:

(60) The catalytic activity of the enzyme was measured spectroscopically following the change in the absorption spectrum due to the production of quinoneimine dye by HRP, as illustrated in FIG. 4A. Typically, 20 μL (1 mg/2.5 mL) HRP and 25 μL (1 mg/mL) SOD, 200 μL (8.125 mg/l mL) 4-aminoantipyrine (4-AAP), 600 μL (79 mg/mL) phenol and 100 μL of 10-100 nM HNPs/NPs were dissolved and mixed with PBS (final volume 2 mL). Then samples were irradiated using 405 nm laser (20 mW/cm.sup.2) while measuring absorption.

(61) Indoxyl acetate assay.

(62) The cholinesterases activity was measured as follows. Typically, indoxylacetate was dissolved in PBS (1 mg/mL) and filtered with a 0.22 μm filter. A cuvette was filled with at least 50 μL of cholinesterase recombinant enzyme solution, enzymes conjugated to nanoparticles or serum. Then the cuvette was irradiated at 405 nm with a 20 mW laser for 7.5 minutes and two minutes after turning it off, 1 mL of indoxylacetate was injected to the cuvette. The kinetics of enzymatic activity was measured by detecting the emission of the product at 473 nm.

(63) Control Experiments for the Photo-Switched HRP Activation:

(64) Control measurements under various conditions were done in order to isolate the light induced HRP activation by NPs excitation from the intrinsic enzyme activity, or non-specific effects of the light excitation. As shown in FIG. 4B, inert atmosphere e.g. absence of oxygen, absence of HNPs or without irradiation, all show negligible activity and product formation in comparison to the product formation under the presence of the formers. This confirms their key role in the formation of the product. Unspecific product formation was also measured in the absence of enzyme. This false-positive signal could be attributed to phenol oxidation by presence of ROS following light irradiation. Note that the phenol oxidation is not mediated through direct charge transfer from the nanoparticles given the absence of this response under inert conditions. Therefore, in order to obtain the effective net contribution of the HNPs to the HRP activation, we subtracted this signal from all data and figures, presented in this frame of work.

(65) CdS—Au HNPs Show Higher Light Induced HRP Activation than CdS NRs:

(66) The photo-induced activity of HRP was also investigated with CdS based nanosystem as photocatalysts. Comparison between CdS—Au HNPs and CdS NRs showed similar trends to those reported in the main manuscript for CdSe/CdS based nanosystem. As shown in FIG. 6, in all conditions, native and in the presence of SOD and hole scavenger (e.g. EtOH), the hybrid system reveals higher catalytic function and efficiency than the semiconductor alone. In addition, the effects of the hole scavenger and the presence of SOD enzyme on the overall HRP activity are repeated in this system as was demonstrated for the CdSe/CdS based systems. The contribution of each of the additives, SOD and EtOH is discussed in detail within the main manuscript.

(67) Hydroxyl Radical Detection:

(68) TPA in PBS (1340 μL, 0.236 mg/mL) was mixed with NPs (100 μL, ˜60 nM) and PBS was added for a total volume of 2 mL. Samples were irradiated using 405 nm laser (20 mW/cm.sup.2) and emission was measured with a Cary Eclipse Fluorometer (Varian Inc.), every minute after excitation at 310±5 nm.

(69) Electron Paramagnetic Resonance (EPR):

(70) ROS were detected by EPR spin-trapping technique coupled with 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap molecule. DMPO was purified in double distilled water, with activated charcoal in the dark. After 15-30 minutes, the DMPO solution was filtered and its concentration was determined spectrophotometrically, using ε.sub.277 nm=8000 M.sup.−1 cm.sup.−1. The solution was stored at 20° C. for no longer than 2 weeks. Samples containing aqueous suspensions of nanoparticles and DMPO (10 mM) were drawn by a syringe into a gas-permeable Teflon capillary (Zeus Industries, Raritan, N.J.) and inserted into a narrow quartz tube that was open at both ends. Then, the tube was placed into the EPR cavity (ER 4102ST) and spectra were recorded, using Bruker EPR 100d X-band spectrometer, during or after illumination with 405 nm 20 mW/cm.sup.2 laser. The EPR measurement conditions were as follows: frequency: 9.77 GHz; microwave power: 20 mW; scan width: 65G; center field: 3458G; resolution: 1024; receiver gain: 1×105; conversion time: 82 msec; time constant: 328 msec; sweep time: 84 sec; # scans=2; modulation amplitude: 2G; After acquisition, simulations of the recorded spectra were performed using an algorithm provided in the WINSIM program, which is available from NIEHS (National Institutes of Health, web site: http://epr.niehs.nih.gov/pest_mans/winsim.html). The results and simulation parameters used for analysis with comparison to relevant parameters reported from the literature are summarized in Table 1 and Table 2.

(71) A time scan mode was used to follow on the increase in signal intensity upon in-situ illumination. The time measurements were taken at static magnetic field of 3448G, which fits the maximum of the second group of peaks from the low field, attributed to the DMPO adduct of superoxide radicals (DMPO-OOH). The parameters were similar to the field sweep mode except: conversion time: 328 msec; time constant: 328 msec; sweep time: 671 sec; # scans=1.

(72) Similar EPR experiments as were presented in the manuscript were also done in the presence of ethanol which can scavenge hydroxyl radicals (FIG. 8).

(73) Oxygen Consumption Measurements:

(74) Oxygen consumption was measured by polarography with a thermostatically controlled (37° C.) Clark oxygen electrode (Strathkelvin 782 Oxygen System; Strathkelvin Instrument Ltd.). HNPs/NRs with PEI and GSH surface coating with the typical concentrations of the HRP activity assay (100 μL of 10-100 nM) in PBS buffer solution were stirred in a closed cell. Under irradiation (405 nm laser 20 mW/cm.sup.2) oxygen consumption was recorded for typically 10 min and calculated as rate of change in the oxygen concentration.

(75) Glutathione as an Alternative Surface Coating:

(76) As described in the phase transfer section above, both hybrids and semiconductor NPs were transferred to aqueous solutions with additional ligands including GSH via ligand exchange mechanism. The GSH coated systems were investigated for the photo-switched HRP activity under the same conditions as for PEI coated systems reported within the main manuscript. Similar behavior was observed for the GSH coated systems with regard to the trends shown for PEI surface coating.

(77) Kinetic Photocatalytic Activity Measurements for HNPs with Different Biological Hole Acceptors:

(78) In order to determine the photocatalytic activity of HNPs in the presence of different biological hole acceptors, hydrogen gas generation via the photocatalytic water reduction reaction was measured. The photocatalysts were dispersed in PBS solution (2 mL; optical density, OD˜1 at 405 nm). The photocatalyst solution was placed in a quartz cuvette and hole scavengers (typically 0.05M), were added to the solution. The solution is purged with argon for 20 min and stirred to achieve oxygen free condition in which ROS formation is suppressed and hydrogen formation occurs. The HNPs were then illuminated with 405 nm 28 mW/cm.sup.2 laser, producing 5.7×10.sup.16 photons/sec. Aliquots of the reaction vessel head space were taken at different time intervals and the hydrogen was detected and quantified using Varian gas chromatograph (model 6820) equipped with a molecular sieve (5 Å) packed column and a thermal conductivity detector. The resulting chromatograms and hydrogen concentration are obtained by the comparison to a calibration curve of known hydrogen amounts.

(79) Cell Culture:

(80) K-562 human bone marrow cell line (ATCC® CCL-243™) were grown in 5% CO.sub.2, 37° C. incubator in medium consisting of RPMI 1640 (Sigma, Cat# R0883) supplemented with 10% heat inactivated fetal bovine serum (Biological industries, Cat#04-121-1A), 1% L-glutamine (Biological industries, Cat#03-020-1A) and 1% penicillin and streptomycin (Gibco, Cat#15140-122). Cells were split every 3-4 days.

(81) Biocompatibility:

(82) The cytotoxic effects of the HNPs were assessed using Live/Dead assay and MTT viability test with K-562 cell line. Typically, cells were incubated with 10 μL of colloidal HNPs and CdSe/CdS nanorods solutions (1-100 nM) and 190 μL of cell medium in 96 well-plate. 21 hr after MTT was added to each well (0.25 mg/mL final concentration). Three hours after that, the cells were extracted by centrifugation at 3000 RPM for 5 min, cell's medium was replaced with DMSO and absorption was measured at 535 nm and 635 nm. The Live/Dead assay was performed 24 hr following the incubation with the NPs according to manufacturer instructions (molecular probes, Cat# L-7013). Working solution was made by diluting the Live/Dead dyes in HBSS buffer. The dye solution was added to the cell pellet followed by 15 minutes incubation in the dark. The cells were incubated for 15 minutes in 4% glutaraldehyde and then observed under fluorescent microscope. Similar Live/Dead assay was done 24 hr following illumination for 5 min with 405 nm 20 mW/cm.sup.2 LED on the cells after their incubation for 1 hour with NRs or HNPs.

EXAMPLES

(83) The following examples are non-limiting, and are provided as a demonstration of the concept of using photo-initiator catalysts in photo-polymerizable inks. The photo-initiator catalysts can be composed of a variety of elements, and their properties are tailored according to the intended application and the printing technology.

Example 1: CdS—Au Photo-Initiator and Measurement of Photo-Initiator Activity

(84) CdS—Au hybrid nanoparticles were grown in a two-step process. In the first step CdS rods were synthesized by seeded growth approach yielding rods with diameters of 5 nm and length of 50 nm. In second step, Au was grown selectively on one apex of the CdS rod, yielding 2 nm sized Au tips. A ligand exchange procedure was applied, through coating by PEI (polyethyleneimine), to render the hybrid nanoparticles dispersible in water.

(85) To determine the polymerization efficiency of nano-photoinitiator catalysts, polymerization kinetics of acrylamide in aqueous solutions with nano-photoinitiator catalysts was studied. Fourier Transform Infrared Spectrophotometer, (ALPHA FT-IR Spectrometer, Bruker) was used in conjunction with platinum ATR single reflection diamond accessory (Sample scans 30; Resolution 4 cm.sup.−1). The polymerization medium comprised aqueous solutions of 15% w/w monomer (acrylamide) with cross-linking monomer polyethylene glycol 600 diacrylate (4% w/w) and nano-photoinitiator catalysts at various concentrations of 1E-7 M to 6E-7M. For comparison polymerization kinetics of aqueous acrylamide solutions with CdS—Au hybrid nanoparticles and CdS nanorods was also studied. Measurements were performed on ˜15 μl of polymerization solution dropped on the ATR diamond. The UV light was radiated onto the sample through a chamber (at 2.5 cm height) centered at the ATR diamond. Monochromatic UV LED (Integration Technology, Oxfordshire, UK) irradiating at 385 nm was used for photo-curing. IR spectra were recorded after every 10 seconds, for a total duration of 180 seconds. The polymerization kinetics was studied by monitoring the FTIR spectra in the range of 1800-800 cm.sup.−1.

(86) The conversion of acrylamide was measured as decay of absorption peaks of methylene group vibrations at 988 cm.sup.−1 (assigned to out-of-plane bending mode of the ═C—H unit) normalized to the C═O stretching peak at 1654 cm.sup.−1 as an internal standard (8). Area under the peak at 988 cm.sup.−1 at different durations of UV exposure was compared with the sample with no UV exposure.

(87) As shown in FIG. 14A, CdS—Au hybrid nanoparticles exhibit significantly faster polymerization than CdS nanorods. The difference between the polymerization kinetics by CdS—Au hybrid nanoparticles and CdS nanorods is statistically significant (t-test; p<0.05).

(88) FIG. 14B shows effect of concentration of CdS—Au hybrid nanoparticles on polymerization kinetics of aqueous acrylamide solutions. Degree of polymerization is proportional to the concentration of CdS—Au hybrid nanoparticles. The nano-photoinitiator catalysts enabled much faster photopolymerization of acrylamide efficiently at concentrations of 6×10.sup.−7 M.

(89) Furthermore, to study the effect of irradiation wavelength, the degree of polymerization of aqueous acrylamide solutions containing CdS—Au hybrid nanoparticles (4×10.sup.−7M) at different excitation wavelengths 385 nm, 405 nm and 450 nm was measured and compared (FIG. 14C). The light intensity of all excitation sources was 25 mW/cm.sup.2. The polymerization is applied at a wide range of wavelengths from the UV to the visible spectra, correlating with the absorbance feature of the CdS—Au. The CdS—Au hybrid nanoparticles showed high degree of polymerization with both UV and visible irradiations.

Example 2: 3D Printing of Model Hydrogel Using Photo-Curable Ink

(90) Preparation of UV Curable Ink Formulation

(91) Composition A:

(92) Weigh 20 g of PEGylated diacrylate 600 (SR610). Add 80 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M. Stir the solution to obtain a clear solution.

(93) Composition B:

(94) Weigh 40 g of PEGylated diacrylate 600 (SR610). Add 60 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6×10.sup.−7M. Stir the solution to obtain a clear solution.

(95) Composition C:

(96) Weigh 25 g of Acrylamide and 25 g of PEGylated diacrylate 600 (SR610) together. Add 50 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M and 5 g of aqueous solution containing CdSe—CdS nanorods. Stir the solution to obtain a clear solution.

(97) Composition D:

(98) Weigh 20 g of N-vinyl 2-pyrrolidone, 2 g sodium dodecyl sulfate and 2 g of PEGylated diacrylate 600 (SR610) together. Add 60 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M. Stir the solution to obtain a clear solution.

(99) Composition E:

(100) Weigh 20 g of N-vinyl caprolactam with 2 g sodium dodecyl sulfate and 2 g of PEGylated diacrylate 600 (SR610) together. Add 60 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M. Stir the solution to obtain a clear solution.

(101) Composition F:

(102) Weigh 20 g of N-Isopropylacrylamide with 2 g sodium dodecyl sulfate and 2 g of PEGylated diacrylate 600 (SR610) together. Add 60 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M. Stir the solution to obtain a clear solution.

(103) Composition G:

(104) Weigh 30 g of 3-acryloxypropyl trimethoxysilane with 2 g sodium dodecyl sulfate and 3 g of PEGylated diacrylate 600 (SR610) together. Add 70 g aqueous solution containing hybrid CdS—Au Nanorods stabilized with PEI at concentration of 6-7M. Stir the solution to obtain a clear solution.

(105) Composition H:

(106) Weigh 40 g of 2-acryloylamido-2-methyl-propane sulfonic acid and 4 g of PEGylated diacrylate 600 (SR610) together. Add 60 g aqueous solution containing hybrid CdS—Au Nanorods stabilized with PEI at concentration of 6-7M. Stir the solution at 30° C. to obtain a clear solution.

(107) Composition I:

(108) Weigh 30 g of Acrylic acid and 3 g of PEGylated diacrylate 600 (SR610) together. Add 70 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M. Stir the solution to obtain a clear solution.

(109) Composition J:

(110) Weigh 25 g of 1,6-hexane diol diacrylate and 2.5 g of PEGylated diacrylate 600 (SR610) together. Add 50 g aqueous solution containing hybrid CdS—Au nanorods stabilized with PEI at concentration of 6-7M and 25 g of isopropanol. Stir the solution to obtain a clear solution. This composition can be used for printing dental composites and structures.

(111) 3D printer (Freeform Plus 39, Asiga, Australia) was used. This printer operates by top-down stereolithography system with digital mirror device and UV-LED light source (385 nm). UV curable ink compositions A-J were used separately for each printing. Using Asiga composer software and .STL file for design, printing command was given with settings shown in Table 3:

(112) TABLE-US-00003 TABLE 3 Setting for a 3D printer used in accordance with the invention. Exposure Time per Layer* (s) 5-180 Burn-In Layers 0-10  Burn-In Exposure Time (s)* 0-240 Layer Thickness [mm]* 0.025-1.00   Light Intensity mW/cm.sup.2 17.5 *These setting were optimized for each printing and structure requirements.

(113) Stable structured hydrogels (FIG. 15) were built using nano-photoinitiator catalysts. Based on the composition used the printed hydrogels had 50-80 w/w water content. Using Composition, structures with complex geometries such as Bucky Ball C180 were 3D printed (FIG. 15). As shown in FIG. 15B, these 3D printed structures fluoresce (glow) under UV illumination (365 nm). Thus, based on these findings, the nano-photoinitator catalysts can be used for high performance photo-polymerization including 3D printing of aqueous systems. This is on top of other printing applications as mentioned above.

(114) Photocatalyitic Activity of Self-Polymerized Semiconductor-Metal Hybrid Nanoparticles (HNPs)

(115) As described, the enhanced photocatalytic function of HNPs to produce reactive oxygen species (ROS) allows the polymerization of hydrogel monomers under visible light irradiation. The HNPs superiority over semiconductor nanocrystals in such process has been extensively discussed in the former sections concerning the relevant of photocatalytic ROS production for modulating enzymatic activity. This advantage is pf high relevance for the photo-polymerization process also holds other advantages over molecular photo-initiators as well. Moreover, the well-known and reported photocatalytic properties of HNPs can be further exploited in the new HNPs-hydrogel matrix. Post polymerized HNPs initiators maintain their photocatalytic properties. Hence, the printed matrix containing the HNPs within the polymer can functionalize it while acting as a unique 3D photocatalyst system. Therefore, in some embodiments, such system can be utilized as photocatalytic membranes for performing redox reactions upon light activation for several applications including water purification or waste treatment.

(116) As demonstration of this novel aspect shown is a photocatalytic dye reduction of methylene blue (MB) by 3D HNPs-hydrogel system as follow:

(117) ##STR00001##

(118) Already polymerized hydrogel with HNPs embedded in it as result of self-polymerization process under visible light illumination was inserted to a solution with MB dyes. Short (10 min) 405 nm irradiation resulted in MB reduction. This was done in ambient atmosphere as well as under inert condition, in both cases the MB was reduced, as shown in FIG. 16, however, under inert conditions the dye was fully reduced in comparison to partial MB reduction at aerobic environment. This photocatalytic activity could be mediated through the production of ROS or even through direct charge transfer of reductive excited electrons (in this case) from the fermi energy level of the metal to the reduction potential of the dye.

(119) A non-limiting example of light-induced ROS formation and polymerization with cadmium free HNPs is provided in FIG. 17. It shows TEM image of ZnSe frames decorated with gold nanoparticles and images showing a polymerized matrix following light excitation of the HNPs.