摘要：金納米棒在光學(xué)、電學(xué)、信息學(xué)和生物醫(yī)學(xué)等領(lǐng)域具有廣泛的應(yīng)用。然而，一些具有特殊要求的金納米棒還不能通過(guò)常規(guī)的方法制備。在本研究中，我們創(chuàng)新地將十二醇（LA）分子引入到傳統(tǒng)種子生長(zhǎng)方法中，成功實(shí)現(xiàn)了具有固定寬度的不同長(zhǎng)徑比（AR）金納米棒（FW-Au NR）的按需制備。此外，通過(guò)合理地選擇相應(yīng)的反應(yīng)條件（如氯金酸和硝酸銀的濃度），可以在130–38.4，109–26.4和16–46 nm范圍之間分別調(diào)節(jié)FW23-Au NRs，F(xiàn)W14-Au NRs和FW6.5-Au NRs （右上角的標(biāo)注數(shù)字表示金納米棒的寬度）的長(zhǎng)度。即，可在一個(gè)較大的長(zhǎng)度范圍內(nèi)調(diào)節(jié)具有固定寬度的金納米棒的長(zhǎng)徑比。并且，在合適濃度的十二醇，0.24–0.30 mmol?L?1范圍內(nèi)調(diào)節(jié)硝酸銀濃度，可以使這些金納米棒的寬度固定在6.5–23 nm之間。另外，實(shí)現(xiàn)FW-Au NRs制備的關(guān)鍵是銀離子和十二醇分子對(duì)分布在金種子晶面上的CTA-Br-Ag+化合物的密度的協(xié)同影響。

關(guān)鍵詞：金納米棒；十二醇；固定的寬度；對(duì)稱性打破效率；有效顆粒數(shù)

中圖分類號(hào)：O648

Synthesis of Different Aspect-Ratios of Fixed Width Gold Nanorods

Abstract： Gold nanorods （Au NRs） have beenwidely used in the optics， electricity， informatics，and biomedical fields in recent years. However， AuNRs with specialized requirements cannot beprepared by conventional methods. For instance， inphotothermal therapy， Au NRs with high aspectratios （ARs） are desirable for increasing tissuepenetration and reducing the burning of human skinduring treatment. However， when their ARs wereadjusted to match the laser used in second nearinfraredwindows （NIR-II）， the length and width ofthe Au NRs simultaneously increased. This increase in width reduces its photothermal conversion efficiency. Unfortunately，tuning the ARs of Au NRs at a fixed width requires complex procedures. In this study， we developed a new seeded-growthmethod to synthesize different ARs of fixed width Au NRs （FW-Au NRs）. To the best of our knowledge， this is the first studyto adjust the length of FW-Au NRs by introducing lauryl alcohol （LA） molecules into the traditional seeded growth method.Moreover， the length span of FW23-Au， FW14-Au， and FW6.5-Au NRs （the superscript numbers denote the width of Au NRsin nm） was adjusted between 130 and 38.4 nm， 109 and 26.4 nm， and 16 and 46 nm， respectively， by judiciously selectingthe corresponding reaction conditions. Notably， the lengths of the Au NRs can be readily achieved at a fixed width over awide range. In addition， their ARs were tuned at a fixed width by adjusting only their length， instead of simultaneouslyvarying their length and width. In addition， their widths were maintained between 6.5 and 23 nm by adjusting [AgNO3]between 0.24 and 0.30 mmol?L?1 in the presence of LA. Furthermore， the synergetic effect of Ag+ and LA on the density ofthe cetyltrimethylammonium （CTA）-Br-Ag+ complexes distributed on the facets of added Au-NP seeds， which can impacttheir symmetry-breaking efficiency （SBE） and the particle number of Au-NP seeds that grow into final Au NRs， is key to thesynthesis of FW-Au NRs. The results of this study offer a flexible and reliable method to tune the length of Au NRs with afixed width and pave the way for achieving an on-demand synthesis of Au NRs， especially for cancer photothermal therapy.

Key Words： Gold nanorods; Lauryl alcohol; Fixed width; Symmetry-breaking efficiency; Effective particle number

1 Introduction

Gold nanorods （Au NRs） have been widely studied in thebiomedical applications such as photothermal therapy ofcancer 1–4， biomedical imaging 5–7 and drug delivery 1，8， in therecent years due to their tunable plasmonic properties，biocompatibility， inherent low toxicity， and diverse surfacemodification. Moreover， the dimension （length width） andaspect ratio （AR） of Au NRs are essential for their performancein general 1，2，9–12. For instance， to improve their photothermalconversion efficiency in the application of photothermal therapyof cancer 13， Au NRs with higher absorption cross-sections arerequired， which is highly depended on their dimension 10，14–16.That is， it is better to prepare Au NRs with a width as small aspossible. On the other hand， near-infrared （NIR） laser inwindow-II （900–1400 nm） is better to increase the tissuepenetration and reduce the burning to skin of human during thetherapeutic treatment 17–19. That is， the position of thelongitudinal plasmon resonance peak （LSPR） of the desired AuNRs have to be well matched with the wavelength of the usedlaser 4，17， which is determined by their ARs. However， when theARs of Au NRs increase to match NIR laser in window-II， boththe length and width of Au NRs simultaneously increase 20，21，which would be disadvantage for photothermal conversionefficiency. In other words， the length of Au NRs cannot becontinuously tuned but remains their width unchanged byadjusting one experimental parameter solely in the traditionalseeded growth method.

For instance， Murphy group achieved the synthesis of Au NRswith approximately the same width （lt; 10 nm） and differentlengths （19–93 nm） by simultaneously changing of reducingagent， modifying the concentration of cetyltrimethylammoniumbromide （CTAB）， AgNO3， Au-NP seeds， and pH values of the growth solution accordingly 22. However， it is still tough toachieve the synthesis of Au NRs with different lengths but withthe same width of bigger than 10 nm. Accordingly， it is still agreat challenge to tune the ARs of fixed width Au NRs by asimple way.

In the traditional seeded growth method used for synthesis ofnormal Au NRs， both their length and width are simultaneouslyincreased （black curve in Fig. 1a） with the decreasingconcentration of Au-NP seeds （[Au-NP seeds]） at a fixedconcentration of AgNO3 （[AgNO3]） 20，21. Accordingly， ARs offixed width Au NRs （red curve in Fig. 1a） cannot be achievedby adjusting the [Au-NP seed]. Moreover， silver ions also canaffect the growth rate of the length and width of Au NRs （Fig.1b，c） 23–27. For instance， when the [AgNO3] is in the range of0.27 to 0.45 mmol?L?1， the widths of the Au NR can keep almostunchanged at the fixed of other reaction conditions （Fig. 1b）.However， their lengths turn to decrease （Figs. 1c and S1（Supporting Information）） 24，28–31，21，32. Therefore， ARs of fixedwidth Au NRs just can slightly be tuned only by adjusting the[AgNO3].

Fortunately， it is recently found that the [AgNO3] in thegrowth solution can determine the symmetry-breaking of theAu-NP seeds and particle number of Au-NP seeds that grow intofinal Au NRs （Neffective） 23，33–35. In the previous work 23，36，37， it hasbeen demonstrated that the Neffective is determined by thesymmetry-breaking efficiency （SBE） of the Au-NP seeds， whichis impacted by the density of CTA-Br-Ag+ complex distributedon the facets of added Au-NP seeds. Thus， it is possible thatwhen typical reaction conditions for synthesis of Au NRs （[Au-NP seeds]， [AgNO3] and [CTAB]， [HAuCl4]， etc.） are all fixed，the ARs of fixed width Au NRs can be tuned by controllingNeffective at the [AgNO3] of 0.27 mmol?L?1 according to the relationship between the [Au-NP seeds] and the size of finalparticles in conventional seeded growth method. As expected， inour previous work 37， it is found that the introduction of fattyalcohols with alkyl chains from 7 to 10 carbon as co-surfactantsall can help silver ions improve the SBE of added Au-NP seedsand further alter Neffective. Moreover， the dimensions of highquality Au NRs are greatly increased， especially at width. Therole of fatty alcohols with alkyl chains from 7 to 10 carbon in thesynthesis of Au NRs with big dimensions is similar to thatreported in literature 36，38. However， when fatty alcohols withalkyl chains bigger than 10 carbons （undecanol or lauryl alcohol）were introduced as co-surfactants， the dimension of Au NRs canbe poorly modulated. This is because the introduction of fattyalcohols with alkyl chains bigger than 10 carbons can result inthe increasing compactness of the CTAB/alcohol mixed micellesled by the strong hydrophobic interactions among them andCTAB molecule， instead of the decreasing compactness of theCTAB/alcohol mixed micelles led by fatty alcohols with alkylchains from 7 to 10 carbons.

Herein， the introduction of lauryl alcohol （LA） molecules intoCTAB are expected to control the compactness of CTABmolecules in the micelles （Scheme 1 and Scheme S1 （SupportingInformation）） 39–41. This is because LA molecules would deeplypenetrate into the micelle core （hydrophobic layers） of theCTAB micelles and reduce the distance between the alkyl chainsof CTAB molecules due to their strong hydrophobic interactionbetween the alkyl chains of them 39–41. In addition， thecompactness of CTAB molecules in CTAB micelles can beadjusted by increasing the concentration of added LA molecules（[LA]） （Scheme 1b–d）. Build on that， the density of CTA-Br-Ag+ complexes distributed on the facets of added Au-NP seedscan be finely increased by increasing [LA]， thus resulting in theincrease in the SBE of added Au-NP seeds and Neffective， andfurther tuning the length of fixed width Au NRs.

In this work， Au NR with tunable length at fixed width of 14nm （defined as FW14-Au NRs） were firstly synthesized underdifferent [LA] at the fixed of other conditions. Next， how the[LA] impact the Neffective are analyzed in detail. Then， Au NRswith tunable length at other fixed width （FW23-Au NRs andFW6.5-Au NRs） were synthesized by changing the [LA] andparticle number concentration of added Au-NP seeds （[Au-NPseeds]） based on the analysis results. Finally， the length span ofAu NRs with the fixed width （6.5， 14 and 23 nm） were furtherbroadened by judiciously altering the [HAuCl4] and [AgNO3] aswell as [HQ].

2 Experimental section

Typically， an aqueous solution of CTAB （0.10 mol?L?1， 5.0mL） and a trace amount of LA （0.015 mL） were sequentiallyadded into a 10 mL glass at room temperature to obtain thegrowth solution under stirring of about 100 min. Then， four typesof aqueous solutions of HAuCl4 （25 mmol?L?1， 0.10 mL）， HNO3（100 mol?L?1， 0.080 mL）， AgNO3 （10 mmol?L?1， 0.15 mL）， andHQ （100 mol?L?1， 0.25 mL） were sequentially added into theresulting growth solution. Finally， 0.060 mL of as-prepared Au-NP seed solution （See more details in Supporting Information）was added to the resulting growth solution. After thoroughlymixing of 2 min， the whole reaction mixture was placed in a water bath for aging at 28 °C. After the aging of 12 h， FW14-AuNRs were separated from the reaction mixture with the aid ofcentrifugation （8000 rcf （relative centrifugal field） × 10 min）.These FW14-Au NR were redispersed into water and centrifugedtwo more times to remove the excess reactants.

Similarly， other types of FW-Au NRs （FW6.5-Au NRs andFW23-Au NRs） with different lengths and widths can be preparedby the same procedure by properly adjusting the concentrationof silver ions， LA， HAuCl4， HQ， and Au-NP seeds were adjusted，respectively （Table S1 （Supporting Information））.

3 Results and discussion

3.1 Synthesis of FW14-Au NRs by adjusting the [LA]

First of all， the optimal [AgNO3] used for growth of fixedwidth Au NRs （FW-Au NR） with a largest range of length in thepresence of LA is determined to be 0.27 mmol?L?1 when the [Au-NP seeds] in the growth solution is 8.4 × 1012 particles?mL?1（Fig. S2）. In the absence of LA， the length and width of Au NRsare about 109 nm and 14 nm （Fig. 2a）， respectively， which isnearly the same to those obtained in the presence of LA with aconcentration below 8.3 mmol?L?1 （Fig. S3a）.

When the [LA] in the growth solution was increased from 0to 8.3， 12.5， and 14.1 mmol?L?1， the length of the resulting AuNRs decreased from 109 to 97， 79 and 69 nm while their widthcan remain at about 14 nm （Fig. 2a to 2d）. Accordingly， theircorresponding ARs can gradually decrease from 8.1 to 7.2， 5.8and 5.0 （Table 1）， which is also in good consistent with thevariation in their extinction spectra （Fig. S4）. Note that sphericalnanoparticles as the by-products would be formed when the [LA]was bigger than 14.1 mmol?L?1 （Fig. S3b）. These results indicatethat the addition of LA into the CTAB growth solution indeedcan impact the length of the Au NRs and keep their width hardlychanged. That is， FW-Au NR can be successfully achieved bysolely varying the [LA] at the fixed other conditions. It is foundthat when [AgNO3] was fixed at 0.06 mmol?L?1 （lower thanoptimal value of 0.27 mmol?L?1） with the [LA] increasing from0 to 8.3， 12.5， and 14.1 mmol?L?1， the length of the Au NRsdecreased from 54 to 44， 39， and 36 nm， while their widthgradually decreased from 31 to 30， 27， and 26 nm， respectively.These results indicate that when [AgNO3] is fixed at 0.06mmol?L?1， FW14-Au NR cannot be achieved by solely varyingthe [LA] at the fixed other conditions. （Fig. S5）

3.2 Role of the LA in synthesis of FW-Au NRs

When the [LA] and other factors （including [HAuCl4]，[AgNO3]， [HQ]， and [Au-NP seeds]） in the growth solution werefixed， both the length and the width of Au NRs can also increasewith the [CTAB] increasing （Fig. S6）. It is self-evident that therelative content of LA in CTAB micelles is decreased when the[CTAB] is increased at the fixed [LA]. These results indicate thatthe compactness of CTAB micellles indeed can impact thesynthesis of FW-Au NRs. As mentioned above， the added LA（including alcohol hydroxyl groups） are expected to deeplylocated in the micelle core （hydrophobic layers） of the CTABmicelles because of the strong hydrophobic interaction betweenthe alkyl chains of LA and CTAB molecules 41. Such interactionswould shorten not only the distance between the alkyl chains， butalso the distance between polar head groups （CTA+） of CTABmolecules in the mixed CTAB-LA micelles. In word， the compactness of the whole CTAB micelles is improved by theintroduction of LA molecules， especially between those polarhead groups （CTA+）. Since CTA+ ions are always complexedwith Br? and Ag+ ions to form CTA-Br-Ag+ complexes， thedensity of CTA-Br-Ag+ complexes in the mixed CTAB-LAmicelles would be higher than that in the pure CTAB micelles.Accordingly， when Au-NP seeds were added into the growthsolution， they would be also stabilized by the mixed CTAB-LAmicelles. And the density of CTA-Br-Ag+ complexes on theirsurfaces would be higher， compared with that in the pure CTABmicelles. Thus， the function of AgUPD on the certain facets ofadded Au-NP seeds would become stronger and the Neffectivewould be increased due to the improved symmetry breakingefficiency （SBE） 23–25.

Accordingly， η （Neffective/Nadded）， which is defined as the ratioof the particle number of Au-NP seeds that grow into Au NRs（Neffective） to the particle number of added Au-NP seeds （Nadded），would increase accordingly with the [LA] increasing. Asexpected， η indeed increases with the increasing [LA] at thefixed other experimental conditions （Table 1）. For instance，when the [LA] was increased from 0 to 8.3， 12.5 and 14.1mmol?L?1 at the fixed [Au-NP seeds] of 8.4 × 1012particles?mL?1， the η increased from 3.3% to 3.6%， 4.5% and4.9% （Table 1） according to the calculated method reported inour previous work 37. The low η value indicate that only a smallproportion of the added Au NP seed （lt; 10%） would grow intothe final Au NRs. It is known that the formed Au NRs can be“shortened” by adding Au3+ ions into their solution 42. Therefore，it is possible that most of the added Au NP seed （gt; 90%） aredissolved in the growth solution because the relative highconcentration of Au3+ ions can react with and the active Au NPseeds during the initial growth stage （Au3+ + 2Au = 3Au+）. Theresults indicate that when [AgNO3] （say， up to 0.27 mmol?L?1）can maximally achieve underpotential deposition of Ag （AgUPD）on the side facets of Au NRs during the growth stage 23，37 （Moredetails in Supporting Information）， the SBE of added Au-NPseeds and the corresponding Neffective indeed can be controlled bythe density of CTA-Br-Ag+ complexes led by the introduction ofLA into CTAB micelles. Therefore， the synthesis of FW-Au NRswith varied lengths can be achieved by solely varying the [LA]at the fixed other conditions.

In addition， when other conditions （including [LA]， [AgNO3]and [CTAB]） are all fixed， the variation in the [Au-NP seeds]would lead to the variation in the density of CTA-Br-Ag+complexes on each Au-NP seed， which would result in thedecrease in the SBE and Neffective. As such， η would also varyaccordingly （Table S2）. For instance， when the [Au-NP seeds]was increased from 2.8 × 1012 to 4.2 × 1012， 8.4 × 1012 and 1.4 ×1013 particles?mL?1 （at the fixed [LA] of 12.5 mmol?L?1）， the ηdecreased from 6.0% to 5.2%， 4.5% and 3.6%， （Table S2）accordingly. Therefore， these results further confirm that theadded LA indeed can achieve the synthesis of FW-Au NRs byfinely affecting the SBE of added Au-NP seeds in the presence of high [AgNO3]. Moreover， the variation in the length of FWAuNRs in the presence of LA is also related to the function ofAgUPD on facets at two ends and sides of the growing Au NRs. Itis known that the order of the deposition of silver ions on eachfacet of Au NRs is as follows： {110} gt; {100} gt; {111} 23–25. Thatis， AgUPD prefers to occur on {110} or higher-index facets ratherthan others 23. Therefore， the function of AgUPD on the side facetsof the Au NRs （mainly enclosed by {110} and {100}） is close tothe maximum under the high [AgNO3] in the growth solutionwhile that on the facets of two ends of Au NRs （mainly enclosedby {111} and {100}） is relatively weak. Therefore， after thecompactness of the mixed CTAB-LA micelles is improved， thefunction of AgUPD on the facets of two ends of Au NRs would bemore obvious because of the loose packing of CTAB moleculesat two ends of pure CTAB micelles while that on their side facetswould remain hardly unchanged （or slight increase）.Accordingly， the growth rate in the length of Au NRs wouldbecome relatively slower while that in the width of Au NRswould remain unchanged （slightly decrease）. As a result， thelength of FW-Au NRs can be tuned by solely varying the [LA]at the fixed other condition. Furthermore， the length of FW-AuNRs may be impacted if the utilization of HAuCl4 in the growthsolution varied after the addition of the LA. It is known thatwhen AA was used as the reducing agent 21，43， only about 15%of the added HAuCl4 become into Au NRs. Therefore， an excessamount of HQ was used in this work to guarantee the 100%utilization of HAuCl4. On the basis of results of ICP tests andcontrol experiments （Fig. S7）， the utilization of HAuCl4 is stillclose to 100%， which are not impacted.

Briefly， the main function of the added LA is to improve thecompactness of CTAB micelles and enhance the density ofCTA-Br-Ag+ complexes distributed on the facets of added Au-NP seed. Accordingly， AgUPD on the certain facets of added Au-NP seeds is impacted， which then improve their SBEs and altergrowth rate of facets at two ends and sides of the growing AuNRs.

3.3 Synthesis of FW23-Au NRs and FW6.5-Au NRs byadjusting the Neffective

In the traditional seeded growth method for synthesis ofnormal Au NRs， the length and width of the Au NRs is usuallyimpacted by the [Au-NP seeds] in the growth solution 22，28，29，31.After a series of control experiments， the [Au-NP seeds] that isappropriate for synthesis of uniform Au NRs in the presence ofLA is determined to be in the range of 2.8 × 1012 to 5.6 × 1013particles?mL?1 （Fig. S8）. In current case， other FW-Au NRs canbe further tuned just by adjusting the [LA] to control Neffective atthe fixed [Au-NP seeds] ranging from 2.8 × 1012 to 5.6 × 1013particles?mL?1. For simplicity， two types of FW-Au NRs weresynthesized by adjusting [LA] at the fixed [Au-NP seeds] of2.8 × 1012 and 5.6 × 1013 particles?mL?1， which can achieve thetuning in the length of FW-Au NRs with a maximal （23 nm） andminimal fixed width （6.5 nm）， respectively （Fig. 3）.

The relationship between the [AgNO3] and width of Au NRs has been shown in Fig. 1. Accordingly， the [AgNO3] used forFW23-Au NRs and FW6.5-Au NRs were adjusted to 0.24 and 0.30mmol?L?1， respectively， which is necessary to control SBE ofadded Au-NP seeds in the presence of LA. As shown in Fig. 3A，F(xiàn)W23-Au NRs with different lengths （widthmaximal = 23 nm） weresynthesized by selecting the [Au-NP seeds] as 2.8 × 1012particles?mL?1 and [AgNO3] as 0.24 mmol?L?1， respectively.With the [LA] increasing from 0 to 8.3， 12.5 and 14.1 mmol?L?1，the lengths of FW23-Au NRs gradually decreased from 130 to110， 99 and 75 nm. Accordingly， their ARs gradually decreasedfrom 5.7 to 4.9， 4.4 and 3.4 （Table S3）. In addition， the positionsof the maximal SPR peaks in their extinction spectra alsogradually blue shift from 970 to 879， 840 and 775 nm （Fig. S9）.Moreover， FW6.5-Au NRs with different lengths （widthminimal =6.5 nm） were prepared by selecting the [Au-NP seeds] as 5.6 ×1013 particles?mL?1 and [AgNO3] as 0.30 mmol?L?1， respectively.Similarly， the lengths of FW6.5-Au NRs gradually decreasedfrom 35 to 30， 27 and 23 nm （Fig. 3B） when the [LA] wasincreased from 0 to 8.3， 12.5 and 14.1 mmol?L?1. In addition，their ARs gradually decreased from 5.2 to 4.6， 4.1 and 3.6accordingly （Table S4）， and the position of the maximal SPRpeaks in their extinction spectra also gradually blue shift from910 to 860， 834 and 788 nm （Fig. S10）. The successfulpreparation of FW23-Au NRs and FW6.5-Au NRs indicates thatFW-Au NRs with a fixed width between 6.5 and 23 nm can beprepared. Moreover， the role of the added LA in theimprovement of the SBE of added Au-NP seeds and Neffective isfurther confirmed by the calculated η value with the increasing[LA] at different [Au-NP seeds] （Tables S3 and S4）.Furthermore， the appropriate [AgNO3] for synthesis of FW-AuNRs in the presence of LA is determined to be in the range of0.24 to 0.30 mmol?L?1 （Figs. S11 and S12）. When the [AgNO3]was higher or lower than the contration range， FW-Au NRscannot be prepared just by adjusting the [LA] （Figs. S11 andS12）. The results further confirm that the synergric effect ofAgNO3 and LA on the density of CTA-Br-Ag+ complexesdistributed on the facets of added Au-NP seeds impact their SBEand Neffective.

3.4 Extending the length span of FW-Au NRs byadjusting [HAuCl4]

As mentioned above， the width of FW-Au NRs can besuccessfully fixed in the range of 6.5 to 23 nm. However， itseems that the span of their length is still limited. It is known thatthe final size of seeds is proportional to the amount of precursorsin the seeded growth method. On the basis of the recipesmentioned above， the length spans of FW-Au NRs were furtherextended by adjusting [HAuCl4] （Figs. 4 and 5， Table S1）. Notethat the [AgNO3] was also varied to remain the fixed widthaccordingly because the ratio of [HAuCl4]-to-[AgNO3] also canimpact the growth rate in the length and width of Au NRs duringtheir anisotropic growth 37.

As expected， when the [HAuCl4] was decreased from 0.46 to0.23 and 0.14 mmol?L?1， the length of FW23-Au NRs decreasedfrom 75 to 55 and 38.4 nm （Figs. 3A-d， 4a and 4b）， respectively.Accordingly， their ARs decreased from 3.4 to 2.5 and 1.7（Table S1） while the position of the maximal SPR peaks in theirextinction spectra also gradually blue shift （Fig. S13）. Similarly，with the [HAuCl4] decreasing from 0.46 to 0.23， 0.18 and 0.14mmol?L?1， the length of FW14-Au NRs decreased from 69 to 53，41 and 26.4 nm （Fig. 4c，d，e）， respectively. Accordingly， theirARs decreased from 5.0 to 3.8， 3.0 and 1.9 （Table S1） while theposition of the maximal SPR peaks in their extinction spectraalso gradually blue shift （Fig. S14）. However， when the[HAuCl4] was increased from 0.46 to 0.58 and 0.69 mmol?L?1，the length of FW14-Au NRs remained fixed at 79 nm， while theirwidths gradually increased from 14 to 16 and 20 nm （Fig. S15）.Therefore， the length span of FW23-Au NRs and FW14-AuNRs can be adjusted between 130–38.4 nm and 109–26.4 nm （Figs. 2， 3A， 4 and Table S1）， respectively by judiciouslyselecting reaction conditions.

Furthermore， when the [HAuCl4] was decreased from 0.46 to0.23 and 0.18 mmol?L?1， the length of FW6.5-Au NRs candecrease from 23 nm to 19 and 16 nm （Figs. 3B-d， and 5a，b），respectively. Accordingly， their ARs decrease from 3.6 to 3.0and 2.5 while the position of the maximal SPR peaks in theirextinction spectra also gradually blue shift （Fig. S16A）.Fortunately， the [HAuCl4] used for synthesis of FW6.5-Au NRscan be higher than that used for syntheses of FW23-Au NRs andFW14-Au NRs. This is because the ratio of gold atoms （NAu） tothe Neffective in the growth solution of FW6.5-Au NRs is rathersmaller than that in the growth solution of FW14-Au NRs andFW23-Au NRs. Thus， the length of FW6.5-Au NRs also canincrease from 30 to 40 and 46 nm （Figs. 3B-b and 5c，d） whenthe [HAuCl4] was increased from 0.46 to 0.58 and 0.69mmol?L?1. As expected， their ARs can increase from 4.6 to 6.0and 7.2 while the position of the maximal SPR peaks in theirextinction spectra also gradually red shift （Fig. S16B）.Therefore， the length span of FW6.5-Au NRs can be adjustedbetween 16 and 46 nm （Figs. 3B， 5 and Table S1） by judiciouslyselecting reaction conditions.

4 Conclusions

In summary， we have successfully synthesized a series of FWAuNRs with a fixed width between 6.5 and 23 nm byintroducing LA in the traditional seeded growth method for thefirst time， to the best of our knowledge. That is， we can tune theARs of fixed width Au NRs by changing their length， instead ofthe simultaneous variation in their length and width. In addition，the [AgNO3] that is appropriate for synthesis of FW-Au NRs isin the range from 0.24 to 0.30 mmol?L?1 just by adjusting the[LA] for controlling the length and width of FW-Au NRs.Moreover， the synergetic effect of Ag+ and LA on the density ofCTA-Br-Ag+ complexes distributed on the facets of added Au-NP seeds can impact their SBE and the final Neffective for synthesisof Au NRs， which is also confirmed by the calculated η valueswith the increasing [LA] at different [Au-NP seeds].Furthermore， the length span of FW23-Au NRs， FW14-Au NRsand FW6.5-Au NRs can be adjusted between 130 and 38.4 nm，between 109 and 26.4 nm， and between 16 and 46 nm，respectively， by judiciously selecting reaction conditions （suchas [HAuCl4]， [AgNO3]， etc.）. Therefore， our work offers aflexible and reliable method to tune the length of Au NR with afixed width and pave the way to achieve on-demand synthesis ofAu NRs.

Author Contribution： Conceptualization， Haibing Xia;Methodology， Haibing Xia; Validation， Hongpeng He，Mengmeng Zhang， and Mengjiao Hao; Investigation， HongpengHe; Data Curation， Hongpeng He and Wei Du; Writing –Original Draft Preparation， Hongpeng He; Writing – Review amp;Editing， Haibing Xia; Visualization， Hongpeng He; Supervision，Haibing Xia and Wei Du; Funding Acquisition， Haibing Xia.

Supporting Information： available free of charge via the internet at http：//www.whxb.pku.edu.cn.

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國(guó)家自然科學(xué)基金（22072076， 21773142）， 山東省泰山學(xué)者（tstp20221106）及山東大學(xué)基礎(chǔ)研究基金資助項(xiàng)目