Name: facet-to-facet linking of shape-anisotropic colloidal cadmium chalcogenide nanostructures
Uploaded: 2017-08-10T14:00:00.000+00:00
Duration: 9 min 12 s
Description: Watch this Scientific Journal Video about Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures at JoVE.com

这项工作得到了 JCO a * 星 Investigatorship 赠款 (项目号 1437C00135), 一个 * 星科学 & 工程研究理事会公共部门资助 (项目 1421200076), 和一个 jsp-新加坡国立大学联合研究项目赠款 (WBS R143-000-611-133) 支持。

1. 预备前体库存解决方案
<ol><li>
顶级股票解决方案。
	<ol>
<li>在氮气气氛手套中, 将11.84 克硒小球称量成150毫升的圆锥烧瓶, 并在烧瓶中放置一个磁力搅拌棒。</li>
		<li>将100毫升的三氮辛基 (顶部) 加到圆锥烧瓶中, 用橡胶隔膜密封烧瓶。</li>
		<li>搅拌的混合物隔夜在 800 RPM。 注: 一旦硒颗粒完全溶解在顶部, 顶部的溶液 (1.5 米) 就可以用于进一步的反应。</li>
	</ol>
</li>
	<li>
顶部的股票解决方案。
	<ol>
<li>将0.611 克的硫粉称量成20毫升的 single-necked 圆底烧瓶 (RBF), 并在烧瓶中放置一个磁力搅拌棒。</li>
		<li>加入10毫升的硫粉和密封的径向基带橡胶隔膜。放置在氮气的气氛下, 搅拌 800 RPM。</li>
		<li>一旦硫粉溶解, 解决方案是明确的, 使用顶部的股票解决方案在进一步的反应。</li>
	</ol>
</li>
</ol>2. CdSe 量子点的合成
<ol><li>
矿 CdSe 量子。 注意: 请参见5。<ol>
<li>将9克三氮辛基氧化物 (拓扑)、6克十六烷基胺 (HDA) 和0.25 毫升 diisooctylphosphonic 酸 (蒂帕) 混合在三颈径向基函数中。在混合物中加入磁性搅拌棒, 并通过刺穿的橡胶隔膜将温度探头插入 RBF。<ol>
<li>在 RBF 上安装回流冷凝器, 并通过适配器将其连接到 Schlenk 线。在所有 glass-to-glass 接头处应用高真空润滑脂。</li>
		</ol>
</li>
		<li>加气溶液在100° c 为 2 h 在恒定的搅动下在800转每分钟。</li>
		<li>分别将乙酰丙酮镉 (Cd (民航)2)、0.570 克 hexadecanediol (HDDO) 和6毫升 1-十八 (颂歌) 混合在 single-necked 径向基函数中。在120° c 和 800 RPM 的2小时内, 在油浴中加入磁搅拌棒和加气搅拌。</li>
		<li>在排气2小时后, 将三颈径向基函数的温度提高到345° c。同时, 让镉前体混合物冷却到室温, 并在 single-necked 径向基函数中加入4毫升的顶硒库存溶液。</li>
		<li>一旦温度达到345° c, 就迅速将整个镉和硒前驱体溶液注入三颈径向基函数。在注入前驱体溶液后立即取出加热地幔, 并允许反应混合物在室温条件下冷却至室温。</li>
		<li>将15毫升的甲苯添加到原油反应混合物中, 并将合成的量子离心为3分钟, 在 2240 x g 处除去沉淀, 并在回收的上清液中加入过量的甲醇。离心混合物在 2325 x g 为8分钟和放弃上清液。</li>
		<li>将5毫升的甲苯添加到沉淀中, 以分散量子. 利用30毫升甲醇进行量子沉淀的进一步处理周期, 将悬浮液在 2240 x g 上离心3分钟, 丢弃上清, 在5毫升的甲苯中分散量子。<ol>
<li>经过两个处理周期后, 将量子的最小用量 (约2-3 毫升) 分散到甲苯中 (此后称为 "量子数库存溶液") 以供进一步使用。</li>
		</ol>
</li>
		<li>采用20µL 的溶液, 并加入3毫升的甲苯, 确定溶液中量子的浓度。测量 350 nm 的吸光度, 并使用已知的摩尔吸收率在 350 nm6上计算浓度。相应地缩放浓度, 以考虑到库存溶液的稀释。 注: 重要的是要保持积极的解决方案, 并立即删除加热地幔后, 迅速注入的前体溶液在步骤 2.1.5, 以获得高尺寸单分散的量子。</li>
	</ol>
</li>
	<li>
锌-闪锌矿 CdSe 量子。 注意: 请参见7。<ol>
<li>将0.0384 克的氧化镉 (CdO)、0.137 克肉豆蔻酸 (MA) 和5毫升的颂歌混合在三颈径向基函数中。在混合物中加入磁性搅拌棒, 并通过刺穿的橡胶隔膜将温度探头插入 RBF。在 RBF 上安装回流冷凝器, 并通过适配器将其连接到 Schlenk 线。用橡皮隔膜密封剩余的端口。在所有 glass-to-glass 接头处应用高真空润滑脂。</li>
		<li>加气溶液在90° c 下恒定搅拌在 800 RPM。</li>
		<li>分别, 加气0.05 毫升油酸 (OA), 0.5 毫升的油 (OM) 和2毫升的颂歌在 single-necked 径向基函数在恒定搅拌下在75° c 和 800 RPM。</li>
		<li>将镉前驱体溶液的温度提高到250° c, 并保持 15 min. 观察溶液中的颜色变化, 从深褐色到无色, 表明镉酸的形成。</li>
		<li>冷却反应混合物到90° c 并且增加12毫升的颂歌。再加气1小时。</li>
		<li>让反应混合物冷却到室温。加入0.012 克硒粉和加气混合物20分钟。</li>
		<li>在 N2下, 将反应混合物的温度提高到240° c。观察颜色变化, 从无色到黄色在150° c 然后到橙色红色在到达240° c, 意味形成 zb-CdSe 量子。</li>
		<li>在240° c 下长出5分钟的量子, 然后将混合物冷却到室温。将 OM、OA 和滴的脱混合物添加到反应溶液中。在室温条件下使溶液冷却至室温。</li>
		<li>通过在生长液中加入丙酮来处理合成的量子, 直到总体积约50毫升。离心结果悬浮在 2240 x g 为 3 min。随后, 丢弃上清液。</li>
		<li>在沉淀中加入5毫升的甲苯, 以分散量子. 进行进一步的处理周期: 用30毫升的甲醇沉淀量子, 将悬浮液在 2240 x 克上离心3分钟, 丢弃上清, 在5毫升的甲苯中分散量子。<ol>
<li>经过两个处理周期后, 将量子的最小用量 (约2-3 毫升) 分散到甲苯中 (此后称为 "量子数库存溶液") 以供进一步使用。</li>
		</ol>
</li>
		<li>采用20µL 处理后的纳米颗粒, 加入3毫升的甲苯, 确定了 CdSe 量子在股票溶液中的浓度。测量 350 nm 的吸光度, 并使用已知的摩尔吸收率在 350 nm6上计算浓度。相应地缩放浓度, 以考虑到库存溶液的稀释。 注: 对于单分散量子的成功合成, 关键是要确保所有的硒粉加入到反应混合物中, 并且它的所有都不留在径向基函数的颈部和侧。</li>
	</ol>
</li>
</ol>3. CdSe 种子 CdS NRs 的合成
注意: 请参见8。
<ol><li>混合3克拓扑, 0.90 克 CdO, 0.80 克 hexylphosphonic 酸 (HPA), 和0.29 克 octadecylphosphonic 酸 (ODPA) 在50毫升三-颈径向基函数。在混合物中加入磁性搅拌棒, 并通过刺穿的橡胶隔膜插入温度探头。</li>
	<li>在 RBF 上安装回流冷凝器, 并通过适配器将其连接到 Schlenk 线。用橡皮隔膜密封剩余的端口。在所有 glass-to-glass 接头处应用高真空润滑脂。将径向基函数加热至150° c, 并将其置于真空下 1.5 h 至加气, 搅拌在 800 RPM。</li>
	<li>放置1.8 毫升的顶部的股票解决方案成一个10毫升 single-necked 径向基函数和密封它与橡胶隔膜。在甲苯中加入 80 nmol 矿 CdSe 量子, 然后在70° c 真空下除去甲苯。让德加的解决方案在 800 RPM 下搅拌30分钟。</li>
	<li>将含有该 Cd 前驱体的径向基函数置于氮气下, 并将温度升高到350° c。在320° c, 通过橡胶隔膜将1.8 毫升的顶部添加到 RBF。</li>
	<li>在350° c 时, 将含有 w CdSe 量子的顶部溶液绘制成注射器, 并迅速注入含有该 Cd 前驱体的径向基函数。允许的解决方案搅拌在 800 RPM 额外的6分钟, 以允许增长的 NRs。随后, 在环境条件下, 除去加热地幔, 冷却溶液至室温。</li>
	<li>为了处理 NRs 的溶液, 在生长液中加入2毫升的甲苯, 并将整个混合物放入50毫升离心管中。加入30毫升的甲醇。随后, 离心结果悬浮在 2240 x g 为3分钟, 然后丢弃上清液。</li>
	<li>在沉淀中加入5毫升的甲苯, 以分散 NRs. 进行进一步的处理周期: 用30毫升的甲醇沉淀 NRs, 将悬浮液在 2240 x 克上离心3分钟, 丢弃上清, 在5毫升的甲苯中分散 NRs。<ol>
<li>经过2-3 次加工周期, 将 NRs 分散在5毫升甲苯中 (此后称为 NR 溶液), 以供进一步使用。</li>
	</ol>
</li>
	<li>准备样品进行 TEM 分析。5<ol>
<li>将一滴 NR 溶液放在一个覆盖有连续碳膜的300网状铜网格上进行电子显微镜分析。</li>
		<li>用吸附剂纸除去多余的溶液, 在室温下烘干样品。</li>
	</ol>
</li>
	<li>进行 TEM 成像 (和分析), 以获得 nr 的尺寸, 使 nr 的平均体积和每 nr 的摩尔数可以确定。<ol>
<li>采用20µL 处理后的 NRs, 并将其添加到3毫升的甲苯中, 确定了 CdSe 种子 CdS NRs 在股票溶液中的浓度。测量 350 nm 的吸光度, 用已知的摩尔吸收率计算 NRs 的浓度 (假定为由 cd 控制)9。相应地对浓度进行缩放, 以解释库存溶液的稀释。 注: 对于一个典型的合成, NRs 的产量接近 100%, 很少有副产品, 如分支结构。</li>
	</ol>
</li>
</ol>4. CdSe 种子 CdSe NRs 的合成
注意: 请参见8。
<ol><li>混合1.035 克的 CdO, 0.1657 克 HPA, 0.1543 克 n-四酸 (TDPA), 和3克拓扑在三颈径向基函数。在混合物中加入磁性搅拌棒, 并通过刺穿的橡胶隔膜将温度探头插入 RBF。在 RBF 上安装回流冷凝器, 并通过适配器将其连接到 Schlenk 线。用橡皮隔膜密封剩余的端口。在所有 glass-to-glass 接头处应用高真空润滑脂。</li>
	<li>加气溶液在150° c 为 1.5 h 在恒定的搅动下在800转每分钟。</li>
	<li>另外, 混合2毫升的 TOPSe 库存溶液和 10 nmol 的 w CdSe 量子 (分散在甲苯) 在 single-necked 径向基函数和德加在一个油浴在 800 RPM 和90° c 的常数搅拌下, 直到所有的甲苯被删除。</li>
	<li>脱气后, 将镉前体混合物的温度提高到340° c。随着温度的升高, 观察溶液中的颜色变化, 从深褐色到无色, 表明镉-烷基膦酸络合物的形成。</li>
	<li>当镉的前驱温度达到340° c 时, 迅速地将1.8 毫升的顶端注入混合物中, 使温度恢复到340° c。</li>
	<li>在到达340° c 时, 迅速地向三颈径向基函数注入1.8 毫升的顶端硒/CdSe 量子混合物, 温度将降到320° c。在恢复到340° c 时, 保持反应溶液在这个温度为 ~ 3 分钟在去除加热的披风之前和允许解答冷却到室温在环境条件下。</li>
	<li>在反应溶液冷却到室温后, 加入2毫升的甲苯并将整个溶液转移到50毫升离心管上。上离心管与甲醇 (99%) 到50毫升和离心结果悬浮在 2240 x g 为 3 min。随后, 丢弃上清液。</li>
	<li>在沉淀中加入5毫升的甲苯以分散 NRs. 进行进一步的处理周期: 沉淀 NRs 使用30毫升甲醇 (99%), 离心悬浮在 2240 x g 为3分钟, 丢弃上清, 并分散 NRs 5 毫升的甲苯。<ol>
<li>经过2-3 次加工周期, 将 NRs 分散在5毫升甲苯中 (此后称为 NR 溶液), 以供进一步使用。</li>
	</ol>
</li>
	<li>将一滴 NR 溶液放在覆盖有连续碳膜的300网状铜网格上进行电子显微镜分析。用吸附剂纸除去多余的溶液, 在室温下烘干样品。</li>
	<li>进行 TEM 成像和分析, 得到 NRs 的尺寸, 从而确定 nr 的平均体积和每 nr CdSe 的摩尔数。<ol>
<li>采用20µL 的溶液, 并加入3毫升的甲苯, 确定溶液中 NRs 的浓度。测量 350 nm 的吸光度, 并使用已知的摩尔吸收率在该波长6上计算浓度。相应地缩放浓度, 以考虑到库存溶液的稀释。 注: 对于一个典型的合成, NRs 的产量接近 100%, 很少有副产品, 如分支结构。</li>
	</ol>
</li>
</ol>5. CdSe 种子 CdSe TPs 的合成
注意: 请参见10。
<ol><li>混合0.43 克 CdO, 3.8 毫升 OA, 2.2 毫升颂歌, 和0.3 毫升的顶部在三颈径向基函数。在混合物中加入磁性搅拌棒, 并通过刺穿的橡胶隔膜将温度探头插入 RBF。在 RBF 上安装回流冷凝器, 并通过适配器将其连接到 Schlenk 线。用橡皮隔膜密封剩余的端口。在所有 glass-to-glass 接头处应用高真空润滑脂。</li>
	<li>加气溶液在90° c 为 1 h 在恒定的搅动下在800转每分钟然后增加温度到265° c。当温度达到265° c 时, 观察溶液颜色的变化, 从深褐色到无色, 表明油酸镉的形成复杂。</li>
	<li>将温度降低到50° c, 加入1.7 毫升的2米 TOPSe (分别由 sonicating、37赫和 320 W 组成, 在0.316 毫升的玻璃瓶中2毫升的混合物中8克硒粉) 和0.017 克甲基溴 (CTAB)。</li>
	<li>另外, 混合7毫升的颂歌, 0.025 克 CTAB, 0.5 毫升的顶部, 0.75 毫升 OA, 和 100 nmol 锌闪锌矿 CdSe 量子 (分散在甲苯) 在三颈径向基函数。在混合物中加入磁性搅拌棒, 并通过刺穿的橡胶隔膜将温度探头插入 RBF。<ol>
<li>在 RBF 上安装一个冷凝器, 并通过适配器将它连接到 Schlenk 线。用橡皮隔膜密封剩余的端口。在所有 glass-to-glass 接头处应用高真空润滑脂。</li>
	</ol>
</li>
	<li>德加在步骤5.4 中的混合物在90° c 和 800 RPM 下不断搅拌为〜45分钟, 直到所有的甲苯被删除。</li>
	<li>将含有锌-闪锌矿 CdSe 量子的反应混合物的温度提高到260° c。当温度达到260° c 时, 用注射器泵将8毫升的镉前驱体溶液添加到含有锌-闪锌矿 CdSe 量子的混合物中, 其注射速率为0.25 毫升/分钟。添加完成后, 在室温条件下使溶液冷却至室温。</li>
	<li>解决方案冷却到室温后, 将其转移到50毫升离心管。加入40毫升丙酮 (99%) 沉淀 TPs 和离心悬浮在 1340 x g 为10分钟。随后, 丢弃上清液。</li>
	<li>将5毫升的甲苯添加到 TPs 的沉淀中, 以分散它们。进行进一步的处理周期: 沉淀 tps 使用30毫升甲醇 (99%), 离心悬浮在 2240 x g 为3分钟, 丢弃上清, 并分散 tps 在5毫升的甲苯。<ol>
<li>在处理2-3 循环后, 将 TPs 分散在5毫升甲苯 (以后称为 TP 库存溶液) 中以供进一步使用。</li>
	</ol>
</li>
	<li>将 TP 溶液放在一个覆盖有连续碳膜的300网状铜网格上, 用于电子显微镜分析。用吸附纸除去多余的溶液, 在室温下烘干样品。</li>
	<li>进行 TEM 成像和分析, 以获得 TPs 的尺寸, 使 tp 的平均体积和每 tp 的 CdSe 摩尔数可以确定。<ol>
<li>采用20µL 的溶液, 并加入3毫升的甲苯, 确定溶液中量子的浓度。</li>
		<li>测量 350 nm 的吸光度, 并使用已知的摩尔吸收率在该波长6上计算浓度。相应地对浓度进行缩放, 以解释库存溶液的稀释。 注: 对于典型的合成, TPs 的收率为 ~ 80%, 20% 是 bipods 和三脚架。</li>
	</ol>
</li>
</ol>6. 小平面活化和纳米结构的连接
<ol><li>
准备胺 (多哈回合) 股票解决方案。
	<ol>
<li>通过在5毫升的乙醇中加入0.140 克的多哈回合来准备一个股票解决方案。几种的解决方案在37赫和 320 W 为〜5分钟, 以确保多哈裁军部完全解散。</li>
	</ol>
</li>
	<li>
阳离子交换和连接。
	<ol>
<li>在适当浓度下准备1毫升的 NC (NR 或 TP) 解决方案 (请参见表 1)。添加6毫克的 ODPA 到1毫升的 NC 溶液和几种10分钟, 在37赫和 320 W。</li>
		<li>另外, 在适当的浓度 (参见表 1) 中, 在一个小瓶中混合1毫升的裁军部股票解决方案和1毫升 Ag+解决方案。加入一个磁性搅拌棒, 并积极搅拌解决方案在 800 RPM。</li>
		<li>在搅拌时, 将 NC 解决方案的1毫升添加到小瓶中, 并允许响应在相应的时间内进行, 如表 1所示。</li>
		<li>在反应结束时, 停止搅拌, 让溶液分离。提取和除去底水层。将5毫升甲醇添加到有机层中, 使其沉淀出来. 离心瓶3分钟在 2240 x g。</li>
		<li>离心后, 丢弃上清液, 加入1毫升的甲苯, re-disperse 该产品以作进一步的表征。 注: 不同材料的阳离子交换反应发生率不同。表 1总结了合成其他材料和形貌的连接纳米结构的条件集。如前所述, 在每组反应中, NC 溶液 (1 毫升)、水性 Ag+溶液 (1 毫升) 和乙醇-多哈解决方案 (1 毫升) 和 ODPA (6 毫克) 的数量保持不变。</li>
	</ol>
</li>
</ol>

<ol>
	<li>Figuerola, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=End-to-End+Assembly+of+Shape-Controlled+Nanocrystals+via+a+Nanowelding+Approach+Mediated+by+Gold+Domains.">End-to-End Assembly of Shape-Controlled Nanocrystals via a Nanowelding Approach Mediated by Gold Domains.</a> Adv. Mat. 21, 550-554 (2009).</li><li>Tang, Z., Kotov, N. A., Giersig, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spontaneous+Organization+of+Single+CdTe+Nanoparticles+into+Luminescent+Nanowires.">Spontaneous Organization of Single CdTe Nanoparticles into Luminescent Nanowires.</a> Science. 297, 237-240 (2002).</li><li>Kim, D., Kim, W. D., Kang, M. S., Kim, S. -. H., Lee, D. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-Organization+of+Nanorods+into+Ultra-Long+Range+Two-Dimensional+Monolayer+End-to-End+Network.">Self-Organization of Nanorods into Ultra-Long Range Two-Dimensional Monolayer End-to-End Network.</a> Nano Lett. 15, 714-720 (2015).</li><li>Schliehe, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrathin+PbS+Sheets+by+Two-Dimensional+Oriented+Attachment.">Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment.</a> Science. 329, 550-553 (2010).</li><li>Chakrabortty, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Facet+to+facet+Linking+of+Shape+Anisotropic+Inorganic+Nanocrystals+with+Site+Specific+and+Stoichiometric+Control.">Facet to facet Linking of Shape Anisotropic Inorganic Nanocrystals with Site Specific and Stoichiometric Control.</a> Nano Lett. 16, 6431-6436 (2016).</li><li>Leatherdale, C. A., Woo, W. K., Mikulec, F. V., Bawendi, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+Absorption+Cross+Section+of+CdSe+Nanocrystal+Quantum+Dots.">On the Absorption Cross Section of CdSe Nanocrystal Quantum Dots.</a> J Phys Chem B. 106, 7619-7622 (2002).</li><li>Dabbousi, B. O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=(CdSe)ZnS+Core-Shell+Quantum+Dots:+Synthesis+and+Characterization+of+a+Size+Series+of+Highly+Luminescent+Nanocrystallites.">(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites.</a> J Phys Chem B. 101, 9463-9475 (1997).</li><li>Carbone, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+Micrometer-Scale+Assembly+of+Coilloidal+CdSe/CdS+Nanorods+Prepared+by+Seeded+Grwoth+Approach.">Synthesis and Micrometer-Scale Assembly of Coilloidal CdSe/CdS Nanorods Prepared by Seeded Grwoth Approach.</a> Nano Lett. 7, 2942-2950 (2007).</li><li>Shaviv, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Absorption+properties+of+Metal-Semiconductor+Hybrid+Nanoparticles.">Absorption properties of Metal-Semiconductor Hybrid Nanoparticles.</a> ACS Nano. 5, 4712-4719 (2011).</li><li>Lim, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+Synthesis+of+CdSe+Tetrapods+with+High+Morphological+Uniformity+by+the+Persistent+Kinetic+Growth+and+the+Halide-Mediated+Phase+Transformation.">Controlled Synthesis of CdSe Tetrapods with High Morphological Uniformity by the Persistent Kinetic Growth and the Halide-Mediated Phase Transformation.</a> Chem Mat. 25, 1443-1449 (2013).</li></ol>

我们没有什么要透露的。

这项工作中描述的连接技术允许形状各向异性的镉硫纳米粒子, 可以进行阳离子交换与 Ag+ , 以加入, facet-to-facet, 进入组件, 如线性链或分支网络。未能形成分散, 广泛的 facet-to-facet 连接的纳米粒子的组装往往是因为两个原因: (i) ODPA 在 NR 的溶液中没有很好的分散, 可以通过 sonicating 的混合物来解决。《议定书》详细规定的时间;或者 (ii) 所使用的 Ag+的浓度是非的。当 Ag 的+?...

胶体半导体的定向组装为纳米结构的制备提供了一条合成途径, 其物理化学性质是与各自的 NC 构造块的集合总和或根本不同1,2,3,4. 在纳米粒子组装的各种方法中, 定向 attachment-in 的方法在本质上是相互融合的, 它是一种允许间电子耦合的方式。然而, 传统的定向附着通常需要微妙的平衡粒子偶极子-, 配体-和溶剂的相互作用通常难以执行, 并使适用于不同的 NC 系统。
我们最近开发了一个湿化学方法的共价连接形状-各向异性镉硫, 通过引入一个活性无机中间体, 通过选址的核过程。微粒由反应的无机中间领域的自发融合以后连接5。尽管该技术仍然基于面向对象的连接机制, 但需要考虑弱的间交互, 从而使其具有更大的灵活性和控制能力。通过部分阳离子交换过程 (溶液中) 先将其尖端面转换为银硫, 以实现形状各向异性镉硫的连接;其次是选择性去除的配体钝化表面。然后, 通过融合暴露的银硫面, 形成了与之相连的镉硫的组装。
在本协议中, 我们证明了连接技术可以应用于各种形状的各向异性镉硫 (即CdSe 种子 CdS NRs 和 CdSe 种子 CdSe NRs 或 TPs), 产生长线性 NR 链或高分枝 TP网络.这些结果表明, 该技术可以推广到各种 NC 形状和金属硫, 适于银阳离子交换。

<table><tbody><tr><td>Cadmium oxide (CdO), 99.5%</td><td>Sigma Aldrich</td><td></td><td>Highly toxic</td></tr><tr><td>Tri-n-octylphosphine oxide (TOPO), 90 % and 99%</td><td>Sigma Aldrich</td><td></td><td>Technical and analytical grade</td></tr><tr><td>Cadmium acetylacetonate (Cd(acac)&lt;sub&gt;2&lt;/sub&gt;), 99.9%</td><td>Sigma Aldrich</td><td></td><td>Highly toxic</td></tr><tr><td>Hexadecanediol (HDDO), 90%</td><td>Sigma Aldrich</td><td></td><td>Technical grade</td></tr><tr><td>1-octadecene (ODE), 90%</td><td>Sigma Aldrich</td><td></td><td>Technical grade</td></tr><tr><td>Dodecylamine (DDA), 98%</td><td>Sigma Aldrich</td><td></td><td>Toxic</td></tr><tr><td>Cadmium nitrate tetrahydrate ((CdNO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;.4H&lt;sub&gt;2&lt;/sub&gt;O), 98%</td><td>Sigma Aldrich</td><td></td><td>Highly toxic</td></tr><tr><td>Myristic acid (MA), 99%</td><td>Sigma Aldrich</td><td></td><td>Analytical grade</td></tr><tr><td>Octyl phosphonic acid (OPA), 97%</td><td>Sigma Aldrich</td><td></td><td>Analytical grade</td></tr><tr><td>Oleylamine (Oly), 70%</td><td>Sigma Aldrich</td><td></td><td>Technical grade</td></tr><tr><td>Hexadecyltrimethylammonium bromide (CTAB), 95%</td><td>Sigma Aldrich</td><td></td><td>Toxic</td></tr><tr><td>Selenium pellets (Se, 5 mm), 99.99%</td><td>Sigma Aldrich</td><td></td><td>Analytical grade</td></tr><tr><td>Hexadecylamine (HDA), 90%</td><td>Alfa Aesar</td><td></td><td>Technical grade, toxic</td></tr><tr><td>n-tetradecylphosphonic acid (TDPA), 98%&amp;nbsp;</td><td>Alfa Aesar</td><td></td><td>Analytical grade</td></tr><tr><td>Silver nitrate (AgNO&lt;sub&gt;3&lt;/sub&gt;), 99.9%</td><td>Alfa Aesar</td><td></td><td>Analytical grade</td></tr><tr><td>Oleic acid (OA), 90%</td><td>Alfa Aesar</td><td></td><td>Technical grade</td></tr><tr><td>Tri-n-octylphosphine (TOP), 97%</td><td>Strem</td><td></td><td>Analytical grade, toxic, air sensitive</td></tr><tr><td>n-hexylphosphonic acid (HPA), 97%</td><td>Strem</td><td></td><td>Analytical grade</td></tr><tr><td>n-octadecylphosphonic acid (ODPA), 97%</td><td>Strem</td><td></td><td>Analytical grade</td></tr><tr><td>Tellurium powder (Te), 99.9%</td><td>Strem</td><td></td><td>Air sensitive</td></tr><tr><td>Tri-n-butylphosphine (TBP), 99%</td><td>Strem</td><td></td><td>Analytical grade, highly toxic, air sensitive</td></tr><tr><td>Diisooctylphosphonic acid (DIPA), 90%</td><td>Fluka</td><td></td><td>Technical grade, toxic</td></tr></tbody></table>

facet-to-facet linking of shape-anisotropic colloidal cadmium chalcogenide nanostructures

在这里, 我们描述了一个协议, 允许形状-各向异性的镉硫纳米晶 (如纳米棒 (NRs) 和四 (TPs), 以共价键和现场-具体连接通过其端面, 导致聚合物样的线性或分支链.连接过程以阳离子交换过程开始, 其中镉硫的端面首先转换为银硫。其次是选择性地去除其表面的配体。这就导致了镉硫的高活性银硫端面, 自发地融合在一起, 从而建立了间 facet-to-facet 附件。通过明智地选择前体浓度, 可以产生一个广泛的联网网络。通过低分辨率透射电子显微镜 (TEM) 和能量色散 X 射线光谱学进行了结构表征, 证实了银硫域之间的链镉硫。

使用 CdSe 的 cd NRs 作为模型系统, 如图 1(a)中所示, 我们演示了可以使用部分 Ag+交换过程来专门转换 NR 提示到 ag2的小平面。Ag2的小平面是由多哈回合, 它与 ODPA 反应通过酸碱反应形成一个不溶性盐5。这将导致从 Ag2的小平面中移除 "裁军部" 配体, 从而使它们在接触时相互融合, 并形成链接?...

Watch this Scientific Journal Video about Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures at JoVE.com

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

本协议详细介绍了形状-各向异性胶体镉硫纳米晶如何通过它们的端面进行共价键连接。

形状-各向异性胶体镉硫纳米结构的 Facet-to-facet 连接

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be ...

facet-to-facet-linking-shape-anisotropic-colloidal-cadmium

Research

JoVE Journal

Chemistry

7.6K 次观看.  National University of Singapore. 本协议详细介绍了形状-各向异性胶体镉硫纳米晶如何通过它们的端面进行共价键连接。

视频: Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

紧凑型量子点的单分子成像

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

科研

工程学

Revealing Dynamic Processes of Materials in Liquids Using Liquid Cell Transmission Electron Microscopy

The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted significant interests across the research fields of materials science, physics, chemistry and biology. The key enabling technology is a liquid cell. We fabricate liquid cells with thin viewing windows through a sequential microfabrication process, including silicon nitride membrane deposition, photolithographic patterning, wafer etching, cell bonding, etc. A liquid cell with the dimensions of a regular TEM grid can fit in any standard TEM sample holder. About 100 nanoliters reaction solution is loaded into the reservoirs and about 30 picoliters liquid is drawn into the viewing windows by capillary force. Subsequently, the cell is sealed and loaded into a microscope for in situ imaging. Inside the TEM, the electron beam goes through the thin liquid layer sandwiched between two silicon nitride membranes. Dynamic processes of nanoparticles in liquids, such as nucleation and growth of nanocrystals, diffusion and assembly of nanoparticles, etc., have been imaged in real time with sub-nanometer resolution. We have also applied this method to other research areas, e.g., imaging proteins in water. Liquid cell TEM is poised to play a major role in revealing dynamic processes of materials in their working environments. It may also bring high impact in the study of biological processes in their native environment.

We have developed a self-contained liquid cell, which allows imaging through liquids using a transmission electron microscope. Dynamic processes of nanoparticles in liquids can be revealed in real time with sub-nanometer resolution.

在液体中使用液体池透射电子显微镜揭示材料的动态过程

The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted ...

Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach.

A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

硒化镉/硫化镉棒和纳米四脚合成种子

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded ...

化学

Synthesis and Characterization of Supramolecular Colloids

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical properties for applications in photonics, drug delivery and coating technology. Here we present a new family of colloidal building blocks, coined supramolecular colloids, whose self-assembly is controlled through surface-functionalization with a benzene-1,3,5-tricarboxamide (BTA) derived supramolecular moiety. Such BTAs interact via directional, strong, yet reversible hydrogen-bonds with other identical BTAs. Herein, a protocol is presented that describes how to couple these BTAs to colloids and how to quantify the number of coupling sites, which determines the multivalency of the supramolecular colloids. Light scattering measurements show that the refractive index of the colloids is almost matched with that of the solvent, which strongly reduces the van der Waals forces between the colloids. Before photo-activation, the colloids remain well dispersed, as the BTAs are equipped with a photo-labile group that blocks the formation of hydrogen-bonds. Controlled deprotection with UV-light activates the short-range hydrogen-bonds between the BTAs, which triggers the colloidal self-assembly. The evolution from the dispersed state to the clustered state is monitored by confocal microscopy. These results are further quantified by image analysis with simple routines using ImageJ and Matlab. This merger of supramolecular chemistry and colloidal science offers a direct route towards light- and thermo-responsive colloidal assembly encoded in the surface-grafted monolayer.

A protocol for the synthesis and characterization of colloids coated with supramolecular moieties is described. These supramolecular colloids undergo self-assembly upon the activation of the hydrogen-bonds between the surface-anchored molecules by UV-light.

超分子胶体的合成与表征

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical ...

Creating Sub-50 Nm Nanofluidic Junctions in PDMS Microfluidic Chip via Self-Assembly Process of Colloidal Particles

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its transparency. Due to the softness of the PDMS material, however, it is challenging to use PDMS for building nanochannels. The channels tend to collapse easily during plasma bonding. In this paper, we present an evaporation-driven self-assembly method of silica colloidal nanoparticles to create nanofluidic junctions with sub-50 nm pores between two microchannels. The pore size as well as the surface charge of the nanofluidic junction is tunable simply by changing the colloidal silica bead size and surface functionalization outside of the assembled microfluidic device in a vial before the self-assembly process. Using the self-assembly of nanoparticles with a bead size of 300 nm, 500 nm, and 900 nm, it was possible to fabricate a porous membrane with a pore size of ~45 nm, ~75 nm and ~135 nm, respectively. Under electrical potential, this nanoporous membrane initiated ion concentration polarization (ICP) acting as a cation-selective membrane to concentrate DNA by ~1,700 times within 15 min. This non-lithographic nanofabrication process opens up a new opportunity to build a tunable nanofluidic junction for the study of nanoscale transport processes of ions and molecules inside a PDMS microfluidic chip.

We propose a simple self-assembly technique of silica colloidal nanoparticles to create a nanofluidic junction between two microchannels in polydimethylsiloxane (PDMS). Using this technique, a nanoporous bead membrane with a pore size down to ~45 nm was built inside a microchannel and applied to electrokinetic preconcentration of DNA samples.

通过胶体粒子自组装过程创建在PDMS微流控芯片分50纳米的纳米流体结

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its ...

Self-assembling Morphologies Obtained from Helical Polycarbodiimide Copolymers and Their Triazole Derivatives

A facile method for the preparation of polycarbodiimide-based secondary structures (e.g., nano-rings, &#34;craters,&#34; fibers, looped fibers, fibrous networks, ribbons, worm-like aggregates, toroidal structures, and spherical particles) is described. These aggregates are morphologically influenced by extensive hydrophobic side chain-side chain interactions of the singular polycarbodiimide strands, as inferred by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. Polycarbodiimide-g-polystyrene copolymers (PS-PCDs) were prepared by a combination of synthetic methods, including coordination-insertion polymerization, copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) &#34;click&#34; chemistry, and atom transfer radical polymerization (ATRP). PS-PCDs were found to form specific toroidal architectures at low concentrations in CHCl3. To determine the influence of a more polar solvent medium (i.e., THF and THF/EtOH) on polymer aggregation behavior, a number of representative PS-PCD composites have been tested to show discrete concentration-dependent spherical particles. These fundamental studies are of practical interest to the development of experimental procedures for desirable architectures by directed self-assembly in thin film. These architectures may be exploited as drug carriers, whereas other morphological findings represent certain interest in the area of novel functional materials.

Here, we present a protocol to prepare and visualize secondary structures (e.g., fibers, toroidal architectures, and nano-spheres) derived from helical polycarbodiimides. The morphology characterized by both atomic force microscopy (AFM) and scanning electron microscopy (SEM) was shown to depend on molecular structure, concentration, and the solvent of choice.

从螺旋聚碳化二亚胺共聚物及其三唑衍生物获得自组装形貌

A facile method for the preparation of polycarbodiimide-based secondary structures (e.g., nano-rings, &#34;craters,&#34; fibers, looped fibers, fibrous ...

Liquid-cell Transmission Electron Microscopy for Tracking Self-assembly of Nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully understood. We have traced the trajectories of individual nanoparticles using liquid-cell transmission electron microscopy (TEM) to investigate the mechanism of the assembly process. Herein, we present the protocols used for liquid-cell TEM studies of the self-assembly mechanism. First, we introduce the detailed synthetic protocols used to produce uniformly sized platinum and lead selenide nanoparticles. Next, we present the microfabrication processes used to produce liquid cells with silicon nitride or silicon windows and then describe the loading and imaging procedures of the liquid-cell TEM technique. Several notes are included to provide helpful tips for the entire process, including how to manage the fragile cell windows. The individual motions of nanoparticles tracked by liquid-cell TEM revealed that changes in the solvent boundaries caused by evaporation affected the self-assembly process of nanoparticles. The solvent boundaries drove nanoparticles to primarily form amorphous aggregates, followed by flattening of the aggregates to produce a 2-dimensional (2D) self-assembled structure. These behaviors are also observed for different nanoparticle types and different liquid-cell compositions.

Here we introduce experimental protocols for the real-time observation of a self-assembly process using liquid-cell transmission electron microscopy.

用于跟踪纳米粒子自组装的液体细胞透射电子显微镜

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully ...

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

用胶体光刻制备周期性金 Nanocup 阵列

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing ...

Ligand Nano-cluster Arrays in a Supported Lipid Bilayer

Currently there is considerable interest in creating ordered arrays of adhesive protein islands in a sea of passivated surface for cell biological studies. In the past years, it has become increasingly clear that living cells respond, not only to the biochemical nature of the molecules presented to them but also to the way these molecules are presented. Creating protein micro-patterns is therefore now standard in many biology laboratories; nano-patterns are also more accessible. However, in the context of cell-cell interactions, there is a need to pattern not only proteins but also lipid bilayers. Such dual proteo-lipidic patterning has so far not been easily accessible. We offer a facile technique to create protein nano-dots supported on glass and propose a method to backfill the inter-dot space with a supported lipid bilayer (SLB). From photo-bleaching of tracer fluorescent lipids included in the SLB, we demonstrate that the bilayer exhibits considerable in-plane fluidity. Functionalizing the protein dots with fluorescent groups allows us to image them and to show that they are ordered in a regular hexagonal lattice. The typical dot size is about 800 nm and the spacing demonstrated here is 2 microns. These substrates are expected to serve as useful platforms for cell adhesion, migration and mechano-sensing studies.

We present a protocol to functionalize glass with nanometric protein patches surrounded by a fluid lipid bilayer. These substrates are compatible with advanced optical microscopy and are expected to serve as platform for cell adhesion and migration studies.

在支撑脂双层配体纳米簇阵列

Currently there is considerable interest in creating ordered arrays of adhesive protein islands in a sea of passivated surface for cell biological ...

生物工程

Fabrication of Zero Mode Waveguides for High Concentration Single Molecule Microscopy

In single molecule fluorescence enzymology, background fluorescence from labeled substrates in solution often limits fluorophore concentration to pico- to nanomolar ranges, several orders of magnitude less than many physiological ligand concentrations. Optical nanostructures called zero mode waveguides (ZMWs), which are 100&#8722;200 nm in diameter apertures fabricated in a thin conducting metal such as aluminum or gold, allow imaging of individual molecules at micromolar concentrations of fluorophores by confining visible light excitation to zeptoliter effective volumes. However, the need for expensive and specialized nanofabrication equipment has precluded the widespread use of ZMWs. Typically, nanostructures such as ZMWs are obtained by direct writing using electron beam lithography, which is sequential and slow. Here, colloidal, or nanosphere, lithography is used as an alternative strategy to create nanometer-scale masks for waveguide fabrication. This report describes the approach in detail, with practical considerations for each phase. The method allows thousands of aluminum or gold ZMWs to be made in parallel, with final waveguide diameters and depths of 100&#8722;200 nm. Only common lab equipment and a thermal evaporator for metal deposition are required. By making ZMWs more accessible to the biochemical community, this method can facilitate the study of molecular processes at cellular concentrations and rates.

Described here is a nanosphere lithography method for parallel fabrication of zero mode waveguides, which are arrays of nanoapertures in a metal-clad glass microscopy coverslip for single molecule imaging at nano- to micromolar concentrations of fluorophores. The method takes advantage of colloidal crystal self-assembly to create a waveguide template.

为高浓度单分子显微镜制造零模式波导

In single molecule fluorescence enzymology, background fluorescence from labeled substrates in solution often limits fluorophore concentration to pico- to ...