1、ARTICLEPorous cage-derived nanomaterial inks for directand internal three-dimensional printingTangi Aubert1,2,4,Jen-Yu Huang3,4,Kai Ma1,Tobias Hanrath3&Ulrich Wiesner1 The convergence of 3D printing techniques and nanomaterials is generating a compellingopportunity space to create advanced materials
2、 with multiscale structural control and hier-archical functionalities.While most nanoparticles consist of a dense material,less attentionhas been payed to 3D printing of nanoparticles with intrinsic porosity.Here,we combineultrasmall(about10nm)silicananocageswithdigitallightprocessingtechniqueforthe
3、direct3D printing of hierarchically porous parts with arbitrary shapes,as well as tunable internalstructures and high surface area.Thanks to the versatile and orthogonal cage surfacemodi cations,we show how this approach can be applied for the implementation andpositioningoffunctionalitiesthroughout
4、3Dprintedobjects.Furthermore,takingadvantageofthe internal porosity of the printed parts,an internal printing approach is proposed for thelocalized deposition of a guest material within a host matrix,enabling complex 3D materialdesigns.https:/doi.org/10.1038/s41467-020-18495-5 OPEN1Department of Mat
5、erials Science and Engineering,Cornell University,Ithaca,NY 14853,USA.2Department of Chemistry,Ghent University,Ghent 9000,Belgium.3Robert F.Smith School of Chemical and Biomolecular Engineering,Cornell University,Ithaca,NY 14853,USA.4These authors contributed equally:Tangi Aubert,Jen-Yu Huang.email
6、:tobias.hanrathcornell.edu;ubw1cornell.eduNATURE COMMUNICATIONS|(2020)11:4695|https:/doi.org/10.1038/s41467-020-18495-5|11234567890():,;Three-dimensional(3D)printing techniques,or additivemanufacturing technologies,have emerged as an enablingplatform for the bottom-up fabrication of advanced func-ti
7、onal superstructures covering a wide range of materials andapplications,from metals and ceramics to biological tissues andorgans14.Among these techniques,digital light processing(DLP)has become a versatile choice,allowing for the printing ofpolymeric or hybrid materials and employing simple commerci
8、alvideo projectors59.This technique makes use of digital micro-mirror devices(DMDs)to generate pre-programmed ultraviolet(UV)/blue light shapes in a plane.When combined with photo-sensitiveresins,thisallowsfortherapid layer-by-layerbuildingofmacroscopic objects with arbitrary shapes and resolutions
9、downto the micrometer scale911.Recent advances in materials scienceprovide an extensive library of nano-sized building blocks,enabling printed materials and devices with programmableoptical,magnetic,plasmonic,and catalytic properties.For light-based 3D printing purposes,nanoparticles and nanomateria
10、ls aretypically used as llers,blended with polymeric binders6,12,13.These composites are,however,often limited to relatively lowweight fractions of the active ller.This in turn may limit theintrinsic added value of the nano-sized components.To fullyexploit the potential of the combination of nanomat
11、erials and 3Dprinting techniques,we developed a class of functional inks basedon a photoresponsive ligand on inorganic core(PLIC)design14.This approach leverages prior nanomaterials research with itsdevelopment of a large variety of nano-sized building blocksoffering a wide range of properties.Advan
12、ces in our under-standing of the surface chemistry of nanostructured materialshave enabled their functionalization with tailored surfaceligands15.The con uence of nanomaterials synthesis andadvanced additive manufacturing methods now allows materialsscientists and engineers to combine nanoscale prop
13、erties ofmatter with the micro-and macroscale structural control offeredby 3D printing techniques.ResultsFormulation of the cage-based PLIC ink.Here we make use ofultrasmall(about 10nm)silica cages formed around cetyl-trimethylammonium bromide(CTAB)micelles swollen withmesitylene and exhibiting well
14、-de ned pentagonal dodecahedralsymmetry with a single,roughly 7nm diameter internal sphericalpore and 4nm-wide window openings on each face(Fig.1a)16.Their sol-gel synthesis based on hydrolysis and condensation oftetramethylorthosilicate(TMOS)as thesilane precursor providesa versatile platform for t
15、heir subsequent surface functionalizationwith a large variety of commercially available organosilanes17.These cages constitute the elementary units forming a number oftwo-dimensional and 3D mesoporous silica materials18,19.Despite substantial fundamental and technological interest,fewexamples exist
16、of 3D-printed mesoporous materials.To the bestofour knowledge,there are no examplesresulting from the useofinksderivedfromindividualporouscagestructuresformulatedasinks.Most examples of 3D-printed mesoporous materials arelimited to extrusion techniques,often with the need for a calci-nation step to
17、remove the template and generate porosity20,21.Ultrasmall photoresponsive ligand-stabilized porous cages pro-vide an opportunity to formulate innovative PLIC inks for thedirect assembly of predesigned macroscopic functional porousobjects by means of DLP 3D printing.This approach enables thedirect pr
18、inting of mesoporous parts with high pore accessibilityand control over the internal structure.We functionalized silica cages with methacrylate-bearing silanegroups,to allow their use as photoresponsive building blocks inthe 3D printer(Fig.1).To that end,we added 3-(trimethoxysilyl)propyl methacryla
19、te directly to the solution after cage synthesis,but with the structure-directing surfactant micelles still present.Previous studies suggested that as part of their formationmechanism,the cage vertices and struts deform the micellesurface,with positively charged surfactant molecules wrappingthe nega
20、tively charged inner silica cage surface22.As illustrated inthe inset of Fig.1a,this soft-templating approach allows todistinguish between inner and outer cage surfaces23,forcing themethacrylate-bearing silane groups to attach predominantly ontheoutersurface.Thecagesweresubsequentlypuri edbydialysis
21、in an acidic water/ethanol mixture,which was shown toef ciently etch away the surfactant template24,25.Removing thecationic surfactant micelle caused the cages to precipitate,consistent with ef cient cage modi cation with hydrophobicmethacrylate groups.This is further supported by the colloidalstabi
22、lity of the cages after additional washing with ethanol andtransfer into non-polar solvents such as toluene.The methacry-late functionalization was also evidenced by Fourier-transforminfrared spectroscopy(FTIR)analyses of the ink,showing thecharacteristic absorption bands of methyl methacrylate grou
23、ps(Supplementary Fig.1).We scaled up the original silica cagesynthesis16byafactorof30(from10mLto300mL)toenablethe3D printing of large parts.Transmission electron microscopy(TEM)studies(see cage image in Fig.1b from a 300mL batch)did not show any noticeable change in cage structure fromcomparisons wi
24、th smaller batch size-derived materials.This wasencouragingandmaysuggestthattherelativelyfacilewater-basedsynthesis may be further scaled up.To formulate the photo-responsive ink for the 3D printer,we combined the methacrylate-functionalized cages with diphenyl(2,4,6-trimethylbenzoyl)phos-phine oxid
25、e(TPO)as the photoinitiator.Exposing this formula-tion to light using a DLP UV projector(385nm,10mWcm2)locallytriggersthepolymerizationofthemethacrylategroupsandforms robust connectionsbetween constituent silica cagebuildingblocks,creating a mesoporous monolith with programmablegeometry.The cross-li
26、nking of methacrylate groups is supportedby FTIR measurements before and after light exposure(Supple-mentaryFig.1),whichsuggestedadecreaseinthealkenesignalascompared to the carbonyl signal.Direct 3D printing of mesoporous parts.We printed macro-scopic patterns on glass slides using the PLIC ink as i
27、llustrated inFig.1d(2min exposure,light dose 1.2Jcm2).We patterned theprojection of a dodecahedron(Fig.1d),demonstrating the largescale andshape-control capabilitiesofferedbythistechnique.Theprinted part showed high delity to the projection pattern(Supplementary Fig.2).Silica cages were labeled with
28、 an organicdyeduringthesynthesisforeaseofvisualization,whichalsomadethe pattern uorescent under UV illumination(inset Fig.1d).TEM analyses of pieces of the printed part(Fig.1c and Supple-mentary Fig.3)evidenced the porous character of the macro-scopic structure.A closer look at the edge of such a pi
29、ece furthercon rmed that the cage structure was preserved during theprintingprocess(insetFig.1c).Thismethodthereforeenables therapid deposition or 3D printing of porous materials with user-de ned shape.In Fig.1d,the parts were printed from a singleprojection layer,typically resulting in a thickness
30、of about 1mm(Supplementary Fig.4).Photo-rheology measurements(Fig.1e)were performed under similar conditions(1mm ink layer,365nm light source,10mWcm2light power)as for the pattern inFig.1d.Although the light sources have slightly different wave-lengths(385nm vs.365nm),the absorption spectrum of TPO(
31、Supplementary Fig.5)shows that absorbance values at thosewavelengths are close enough to make the experiments compar-able.In Fig.1e,the photo-rheology measurement started with aARTICLE NATURE COMMUNICATIONS|https:/doi.org/10.1038/s41467-020-18495-52 NATURE COMMUNICATIONS|(2020)11:4695|https:/doi.org
32、/10.1038/s41467-020-18495-5|30s stabilization period not displayed in the plot(i.e.,UV irra-diation starts at t=0s).The PLIC ink showed a behavior similarto typical photo-polymerization of acrylate polymers.The gelpoint,de ned as the crossover between the storage modulus(G)and loss modulus(G),was re
33、ached after about 15s(light dose0.15Jcm2)of UV irradiation(inset Fig.1e).In addition,the inkalso shows a shear-thinning behavior(Supplementary Fig.6),with viscosities below 0.1Pas for shear rates above 10s1,wellbelowthe5Pasrecommendedviscosityforrapidrecoatingofinklayers26andeven belowthe viscositie
34、s ofatypicalcommercial 3Dprinting resin.These rheological properties allow for facile bedhomogenization during printing and enable the fabrication ofmultilayer 3D structures(Fig.1f),even with self-supporting fea-tures(Supplementary Fig.7).In Fig.1f,the 3D structure wasprintedfromtenlayersof0.9mminth
35、icknessusingahome-builttop-down setup(see“Methods”section for projector speci ca-tions).Although the photo-rheology experiment(Fig.1e)indicated a gel point at 0.15Jcm2,the light dose was set to0.6Jcm2for each layer(i.e.,1min exposure time)to ensuregood structural cohesion of the 3D part.With the DLP
36、 technique,a possible material-dependentlimitation of the printing resolution would be light scatteringfrom the ink itself27.Analysis of the as-prepared ink by UV-visible(vis)spectrometry revealed no signi cant scatteringaround the projector wavelength of 385nm,however,with about95%transmission thro
37、ugh 1cm of ink(Supplementary Fig.8).Besides light scattering,the printing resolution for DLP is mostlyhardware related.It mainly depends on the projected pixel size,which is a combination of DMD speci cations and optics,and onthe ability to accurately control the z position of the stage.Thehome-made
38、 setup that was used for the prints in Fig.1d,f andSupplementary Fig.7 is not optimized to achieve high resolution.To demonstrate that the highest resolutions can be reached usingthe PLIC inks reported here,we used another DLP projector,namely a Wintech PRO4710 with an orthogonal DMD for aprojected
39、pixel size of 35.5m(see“Methods”section forprojector speci cations).A thin layer of the PLIC ink wasapplied on a glass substrate and a pattern with single pixelresolution was projected,i.e.,each of the white squares in Fig.2acorresponds to an individual micromirror of the DMD.Observations with an op
40、tical microscope(Fig.2b)showed high delity of the printed pixels with the pattern,evidencing that thehardware limit can be reached with the PLIC inks.In addition,this high resolution carries over to large areas(Fig.2c).Microstructure of the cage-based printed parts.The combina-tion of PLIC inks and
41、DLP printing provided several knobs totune the internal microstructure of the printed parts,either bychangingtheliganddensityorbyvaryingthelightdose.Largeandfree-standing honeycomb structures were printed from cageswithdifferentmethacrylate ligandcoverage(Fig.3a,lightdose1.2Jcm2).Interestingly,adjus
42、ting the ligand coverage allowedcontrol of the printed material density.Comparison of partsfabricated from low and high ligand-coverage PLICs revealed apositive correlation between methacrylate silane ratio anddensity.For example,while sharing virtually identical dimensions,a bdefUVc5 nm100010002 04
43、 06 0Step time(s)80 100 120G storage modulusG loss modulusStorage/loss modulus(Pa)1013 14 15 16 1710.15 nm 5 nm50 nm50 nmOO Si:Fig.1 Direct printing of mesoporous parts.Illustration(a)and TEM image(b)of silica cages functionalized with methyl methacrylate groups(inset ina:illustration of a cage stru
44、t partially wrapped by the surfactant micelle surface,in blue).c TEM image of a piece of a printed part(inset:zoom in,redarrowspointtoclearlyvisiblecagestructures,scalebar10nm).dPhotographofapatternprintedonaglasssubstratewithdye-functionalizedcages(inset:photograph under UV illumination).e Rheology
45、 measurements,including storage modulus(G,blue line)and loss modulus(G”,red line),of the PLIC inkunder UV irradiation(inset:zoom in to the region around the gel point).f Photograph of a multilayer 3D structure printed from the PLIC ink.NATURE COMMUNICATIONS|https:/doi.org/10.1038/s41467-020-18495-5
46、ARTICLENATURE COMMUNICATIONS|(2020)11:4695|https:/doi.org/10.1038/s41467-020-18495-5|3as precisely measured by confocal microscopy(SupplementaryFig.4),the parts in Fig.3a weight 22.4 and 12.9mg for the high-coverage and low-coverage samples,respectively.Thus,the high-coverage part isabout1.7timesden
47、serthanthelow-coverageone.We attribute the difference in density to varying internal micro-structures.These parts had similar speci c surface areas,speci-cally 438 and 450m2g1for the high-and low-coverage parts,respectively,as determined by the BrunauerEmmettTellermethod.The hystereses of the nitrog
48、en sorption measurements(Fig.3b)show important differences in adsorptiondesorptionbehavior.The broader hysteresis of the high-coverage part sug-geststhataccesstomesoporesismorerestrictedthaninthecaseofthe low-coverage part,for which mesopore access is facilitated bythe presence of more interstitial
49、space and channels.Theopticalpropertiesoftheprintedstructure arealsosensitiveto the methacrylate silane ratio of the PLIC ink as illustrated bythe photographs in Fig.3a.Although after drying the high-coveragepartappearsrelativelytransparent,thelow-coverageoneis more translucent due to light scatteri
50、ng by larger pores.Toinduce signi cant Mie scattering,these larger pores likely are afew tens of nanometers in size and correspond to interparticlepores,because the intraparticle pores,i.e.,cores of the cages,are10nm16.Theseinterparticleporesaremostlikelyformedduringthe drying steps,because the low
