Researchers led by Dr. Chao Zhang from
Peking University seem to have found a solution.
The group made use of an oxidation-reduction
reaction between silver nitrate and ascorbic acid
(a form of vitamin C) [1]. This reaction deposits
a shel l of si lver metal onto gold nanorods.
Fi rst, researchers mixed gold nanorods with
cetyltrimethylammonium chloride (CTAC). This
colloidal solution is initially red in colour. As the
reaction progresses and deposits more silver
onto the nanorods, the mixture releases shorter
wavelengths – orange, yellow, etc. – until the
colour turns green. The team deliberately chose
green as the final colour, as opposed to shorter
wavelengths such as blue and violet, for easier
identification.
By altering the pH and concentrations of
chemicals, Dr. Zhang was able to program
the mi xture for use in different per ishables.
Furthermore, adding weak acids such as acetic
acid and lact ic acid cou ld ensure proper
synchronization between food spoilage and
mixture colour change at a range of temperature
cond i t i ons . Therefo re, the ch ronoch romi c
changes of the mixture serve as a true “time-
temperature indicator”: red means the food is
very fresh, orange and yellow show varying levels
of bacterial growth, and green means the food is
no longer suitable for consumption. For simplicity
in real-life applications, Dr. Zhang’s team solidified
the liquid indicator into an agar hydrogel that is
sealed from external oxidation.
“Hang on,” you may ask, “is the indicator
both safe and affordable?” Gold is an iner t
metal. While silver nitrate can be corrosive, the
addition of CTAC renders it harmless by forming
a silver chloride precipitate. Other chemicals
added have low toxicity and are often found
in medicines, cosmetics and even food. As for
affordability, gold and silver, though expensive,
are used at very low concentrations and do not
contribute much to cost. In fact, each smart tag
costs less than HKD $0.20 to manufacture.
The development of this smart tag has far-
reaching implications. Not only can it tell us
whether food has gone bad, but it can also be
applied to other industries with expiry dates such
as pharmaceuticals and cosmetics. Hence, we
may see little red gels on our perishables in the
near future.
消
費者往往是憑包裝上的「到期日」來決定食物是
否適宜食用。可是，要準確推算到期日並不容易，再加上在
製作和運輸過程中，會遇到無法預計的變化，以致日期不
能充分反映食物變質的程度。很多人便曾遇到打開包裝，卻
發現沒到期的牛奶已變質發臭的情況。如果有一種標籤，
讓我們不需要打開包裝，就可以得知裏面的食物有多新鮮，
我們可以避過臭味和食物中毒。這樣不是很好嗎？
除了在製作過程中受到細菌污染之外，食物變質還有兩
個主要原因：細菌增長和食物分解，兩者都與儲存溫度有
關，隨著溫度上升而加快。如果我們製作一種標籤，可以模
擬這兩個過程的速度，然後將標籤黏在食物包裝外，讓它
們經歷同樣的溫度變化，那麼消費者從標籤就可以看出食
物是否有變壞。
可是，要製造這樣的標籤談何容易：如何做到標籤正確
反映食物的品質變化？如何做到標籤清楚易明？最重要的
是如何把成本降低到大眾負擔得起的水準呢？
北京大學的張超研究員及他的科研組似乎找到了答
案。他們利用硝酸銀和抗壞血酸的氧化還原反應，將銀金
屬沈澱在金納米棒上。研究員先把金納米棒放到十六烷基
三甲基氯化銨（簡稱CTAC）裏，製成紅色的膠體懸浮液。氧
化還原反應會產生越來越多的銀金屬，發出的可見光波長
隨之而縮短，顏色從紅色逐漸轉為橙色、黃色等，直到標籤
最終變成綠色。雖然藍色和紫色的波長比綠色更短，但肉
眼對綠色特別敏感，所以研究員選擇綠色為最終的顏色。
透過改變化學成分的酸鹼度和濃度，張博士可以將標
籤應用在不同的食物上。加入醋酸和乳酸等弱酸更可以讓
食物的腐化和標籤顏色的改變，在不同的溫度下均能以相
同速度進行。標籤可以作為”時間-溫度顯示器”，反映食
物的品質：紅色代表食物非常新鮮，橙色和黃色代表不同程
度的細菌增長，綠色則意味著食物已變壞。為了方便使用，
張博士把液體標籤製成密封的瓊脂水凝膠，免受外來氧化
反應影響。
你可能會問：「標籤是否安全？價錢大眾化嗎？」其
實，金是惰性金屬，不容易產生化學反應。硝酸銀可能有腐
蝕性，可是CTAC會將銀離子轉變成氯化銀，轉化成無害物
質。其他化學成分不含高毒性，在食物、藥物和化妝品中經
常出現。至於價錢方面，雖然金和銀是貴價金屬，可是使用
的濃度較低，不會對成本構成太大影響。每一塊智能標籤
的製作成本低於港幣＄0.20。
智能標籤的開發有廣泛意義，不單可以告訴我們食
物是否變質，還可以應用在其他非耐用品，如藥物以及化妝
產品等。不久將來，我們可能會在包裝上看到小小的紅色標
籤。
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